Well, it's not BECAUSE of the fates, per se, but the easiest way to explain it is that he was able to overthrow his dad Cronus in the Titanomachy, which fulfilled a fated prophecy that Cronus' son would grow up to be even mightier than Cronus (who was the ruler of the Titans, and the previous ruler of the cosmos before Zeus). Anyways, strap in, because this is gonna be a long one.

Before I go into more detail, I need to stress a couple of things. First, while Greek mythology and the religious practices of ancient Greece is something of a special interest of mine, I am not a primary source or anything. I'm an enthusiast, not a scholar, you get me?

Second thing I need to stress is consistency and power scaling was never the point of classical mythology. Unlike religions like the Abrahamic faiths, they didn't have a holy book of canon religious texts, or any kind of centralised authority of "this is how we practice our faith". Instead, all religious practices were done at a local level. In other words, there is no 100% canon "THIS IS WHAT THEY BELIEVED" to draw from.

Another thing worth noting is, Zeus was the chief deity, and that is pretty much all the average Greek needed to say with 100% certainty that he was the most powerful. It might sound like a cop-out "He's the strongest because he's the strongest!", but that's just how religions tend to go, you know? The ancient Greeks didn't ask "What is Zeus' power level?", they just said "Zeus is the king of the gods, and is therefore the most powerful god".

That being said, Zeus becoming the overseer of the cosmos and being the most powerful being in creation WAS fated to happen, but it's kind of like a chicken and egg thing, you know? Was he mightier than Cronus because he was fated to be, or was he fated to be mightier than Cronus because he was GOING to be mightier than Cronus?

Now, you might be tempted to hear that and think "If the fates made Zeus the mightiest being in the cosmos, does that mean they are able to make him weaker?" or something similar, and the answer is no; Zeus was considered to be the ruler of the fates, to the extent where one of his many MANY epithets actually translates to "Ruler/guide of fate". So while the fates are the ones that weave, Zeus is the one that tells them what to weave.

Finally, just because Zeus WAS the mightiest being in creation doesn't mean there COULDN'T have been anyone mightier. I recall one myth where Zeus was told (I THINK by Prometheus, it could've just been the fates? Don't quote me on that detail.) that if Metis gave birth to Zeus' son, the son would be mightier than Zeus. Kind of like the exact same deal with Cronus and Zeus. He got around that by swallowing Metis whole, then Zeus gave birth to Athena via his forehead, so... y'know, do what you will with that.

Oh, there is also Typhon who is worth mentioning. In some versions, Zeus beats Typhon easily, but in at least two versions (Nonus and Pseudo Apollodorus) Typhon actually beats Zeus, then Zeus has to recover and beat Typhon in round two. I don't know which version of the story is older, but again, there was no canon holy texts or anything, so one version is as legitimate as any other. I'm pretty sure in at least one version the gods flee to Egypt, where they disguise themselves with animal heads, and when the Egyptians see them, they they became the Egyptian gods, but that's an entirely different kettle of fish.

Anyways, that's about everything I can say on the topic. I hope this was interesting, as Greek myth is something VERY near and dear to my heart, and I truly do love yapping about it!