The lore reason is essentially that defeating Sauron was mankind’s coming-of-age story (the age of elves was ending, and mankind was set to take over control of middle earth), and having a bunch of maiar come in and wreck Sauron wouldn’t teach men to stand up for what’s right. Instead, Eru told Manwë to send the istari to guide men and elves to defeat Sauron on their own

The “real” reason is that it wouldn’t be a very good story if Manwë just sniped Sauron from the hidden West with magic