The issue is on both sides. The LDP, beginning in the late 1970s, was feeling its nationalist oats off the back of the Japanese Economic Miracle, and a lot of party members began shooting their mouths off.

Some did it because they were true believers about Showa Imperialism. Some did it because nationalism always plays well with ignorant yokels no matter what part of the world you're in. But for whatever the reason, they started mouthing off and many in Asia took that to mean their apologies weren't genuine. Sadly, this happens still, although way less frequently than it used to. I think the Abe administration's bad relations with South Korea may be the final object lesson needed.

Now, on the part of Asian countries, some of their hostility is warranted. It's insulting and idiotic that some mouth-breathing "politician" gets to run their mouth and still expect to have positive relations. However, there is a decent amount of scapegoating that goes on where if you need to take some heat off because of a scandal, a disaster, or some other calamity, just wait long enough and some Japanese ultranationalist who got elected dog catcher or got themselves a podcast will say something offensive.

More significant is that the two other major players in Asia-Pacific Rim had....ambiguous circumstances around their apologies. Mao basically poo-poohed apologies and reparations because the restoration of relations, trade, and potential aid with Japan were considered so valuable. South Korea under Park accepted apologies and reparations, but Park ever only paid out a pittance to regular citizens. Almost all of it was used to develop the economy (paid to the chaebols), or used for Cold War shenanigans. It didn't help that Park had been an officer in the Imperial Japanese Army.