I think the humans, elves, and dwarves were not only weaker/less armed/less trained at the time of the movies vs the initial fight, but also less united/allied
What a shit name for a war tbh. "Yo guys this was fun, but this is the last time we're getting together" - elves probably. Or maybe they meant last as in "previous", but that's just asking for it to get outdated.
Middle Earth's magic is in decay by the third age. The elves are literally leaving it, and there are way less of them. Men also only live to ~80 instead of living for hundreds of years like they did in Numenor. Middle Earth eventually becomes a world like ours.