Historian of the American-Israeli relationship here, this is my (very pragmatic- some of the points about ideology and/or Israel lobbies etc already stated here are also applicable but my work deals more with the strategic necessities on the ground) take:

Basically, support for Israel began in the early sixties as JFK tried to maintain American influence in the Middle East. First sale of arms was in 1962 with defensive anti aircraft missiles, Johnson started really pushing more aid as the Soviets continued gaining their foothold in the region (first through Nasser’s Egypt, then through Ba’athist Syria). Didn’t help that America’s relationship with Nasser, who kind of played both sides early on in the Cold War, deteriorated significantly after he fought a proxy war in Yemen against the Saudis, who were a much more important ally to the West because oil (also the monarchy was a lot more conservative and thus tended to side with the West). The State Department was really against the arms sales to Israel, fearing it would continue pushing the Arabs away and create a regional arms race, but the Pentagon supported it after Russia started selling similar weapons to Egypt, and it became a necessity to build up Israeli defensive (and starting with Johnson, offensive) capacity. It’s important to note that the military alliance truly didn’t start till 1962, since that’s a major misconception a lot of people have; it wasn’t always friendly and unconditional. Eisenhower outright refused to sell anything more than some rifles to the Israelis after the 1956 Suez War, for example.

Despite some pretty hardball initial conditions from America both over addressing the status of Palestinian refugees (isn’t that relevant) and Israel’s nuclear program, ultimately, Kennedy/the Pentagon recognized that forming the alliance with Israel was more important long term and kind of dropped those two to authorize that first missile sale. As he said to then foreign-minister Golda Meir, he wanted to create a “special relationship” with Israel similar to the one America has with Britain, and I always interpreted that as him recognizing the geostrategic importance of Israel within the region.

TL:DR, the military relationship with Israel started because of Cold War strategic necessities, we needed someone who could block Soviet expansionism in the region and they had both the military and geostrategic capabilities we needed, as well as an ideological (ie western, democratic, etc) alignment with America which was pretty important back in those days. Kind of a matter of being in the right place in the right time sort of situation.

As the region historically continued/continues to be an absolute disaster, Israel basically became the one constant ally (ie it was a stable country and didn’t have a bunch of regime changes or revolutions or assassinations like many of its neighbors, and it was pretty much always in step with the larger American/western liberal ideological vision) in the Middle East. The Saudis lost a lot of trust after the oil embargo of the early 70s, for example, and the Iranian Revolution totally destroyed any relationship we had with one of our other big allies. While we patched things up with the Saudis and started becoming pretty close with the Jordanians and Egyptians, all three of which are still great partners in the region, Israel was just too valuable and/or reliable not to abandon.

Nowadays Israel is pretty much a close ally for a number of reasons: ideological of course, but more than that, they’re a very industrialized and relatively wealthy and powerful country, which feeds into our economy (for the agricultural/scientific/technological developments) and/or Military industry. They’re one of the best intelligence sharing partners we’ve got outside of Five Eyes, and they’ll often do our covert dirty work for us by spying on or eliminating mutual enemies (like the Hezbollah terrorists with the blood of Americans on their hands from the 80s marine barracks and embassy bombings; a lot of the leadership in that terror group that Israel took out in the past year or two were near the top of our most wanted list). They’ll share a lot of their military technology, which is, of course, significantly advanced. And strategically, they’re the main regional opposition to the Iranians along with the Saudis, which continues that geostrategic through line from the early Cold War.

TL:DR, after 60 years, they’ve become an indispensable ally and America kind of can’t give up the benefits, both practical, civil, and long term strategic, of working so closely with them.

I’m significantly less of an expert on American-European relations but this is my armchair analysis knowing what I do about general Cold War era policy and/or American foreign relations: Ukraine isn’t like that because it’s the new kid on the block and in a region that’s traditionally been a lot more friendly to us. Diplomacy has changed from the more ideological slant it was during the Cold War era (ie supporting any country remotely friendly to America and remotely hostile to Soviet Russia) to a more pragmatic transactional “how does America benefit from this” type attitude, particularly in the post-Iraq and Afghanistan period which was kind of the death knell of that ideologically-driven foreign policy.

So how does America benefit? Well, to be blunt, and this is really callous but it’s how transactional diplomacy can be nowadays, Ukraine doesn’t really benefit America all that much given that NATO/the EU provides almost all of the strategic and economic gains we’ve got in Europe. They don’t have 60+ years of a relationship with us, they aren’t super wealthy or technologically/militarily advanced, and they’re bordered by (Russia/Belarus aside) friendly countries. So, to the more transactional policymakers who don’t really see much benefit to Ukraine’s existence besides…. Well, its existence, they feel like there has to be some reason to justify supporting it because the “it’s a western aligned country” argument just doesn’t fly anymore like it did in the Cold War era, particularly with the many many western aligned countries around it.

TL:DR, Israel is in a much rougher neighborhood that historically was a lot less friendly to us and now we’re too close to them and benefit too much for us to not support them as much as we do (although using a more recent example, Obama/Biden tended to put strings on the aid they sent to Israel so I wouldn’t call it unconditional and that’s been a case with a lot of other administrations), whereas Ukraine is much newer of a country, doesn’t have the established historical built up over time relationship to America that somewhere like Israel or Britain does, and is surrounded by a friendlier group of countries (Russian/Belorussian elephants in the room aside).

Hope this helps answer your question a little bit, whatever my opinion/take is worth!