Honestly, I always considered the preponderance of humans in Starfleet to come down to a very simple explanation: more humans join Starfleet, both proportionately and in absolute numbers, than the other member worlds.
Early on, it could be that Earth was trying to make up for its perceived youth and inability to act independently of Vulcan. Then it eventually turned into the "we look to better ourselves" post-scarcity thing, and Starfleet is the place to go to improve yourself and make the galaxy a better place. Then, alongside that, I'd always imagined that the other early member worlds of the Federation were much less expansionist and/or less willing to plonk an unsupported colony down on a marginal world during the 22nd and 23rd centuries, so that by the time it gets to the 24th century there are flat out more human colonies in the Federation than there are other member world colonies.
Like, a recurring thing in Star Trek is how the "Hat" of humans is drive. Exploration of the unknown, settling a world without much aid, operating a vineyard in the 24th century without a lick of technology beyond that of the 19th century, insisting on peaceful cooperation when it would be so much easier to "do the hard thing"? Romulans are schemers, Klingons are warriors, Vulcans are logical, and Humans are stubborn.