According to The Hollywood Reporter, Joaquin Phoenix passed on the role, while "Girls" star Adam Driver was approached but had a scheduling conflict.
It's clear Warner Bros. wanted to go younger with Luthor, as both Eisenberg and Driver are 30 (Phoenix is 39). Now, the super villain will match up in age with Henry Cavill's Superman. At 41, Ben Affleck's aging Batman will be the old guy of the bunch.
Interesting side note: Sources told THR that "Breaking Bad" star Bryan Cranston was never in consideration.
As THR points out, this is more in line with recent graphic novel and television versions in which Luthor and Clark Kent knew each other as children. All director Zack Snyder teased about his Luthor was that Eisenberg would take the character "in some new and unexpected directions."