My guess is Feige and Whedon didn't feel confident in doing a straightforward adaptation of Adam. He's an obscure character (sadly) and would have really required his own movie to be properly explored. I disagree with what they did but after digesting it since Thursday night I think there's hope for Adam to end up where he needs to be for Infinity Wars.
Anyway, I posted this in the other thread. Its a list of red flags pointing to Adam that showcase how Vision was just adapted visually.
-Vision isn't born a mechanical android but rather a bio-engineered being with organic flesh by Dr. Cho (like Adam by the High Evolutionary)
-His consciousness comes from the gem, making the two symbiotic (like Adam and the Soul Gem)
-Thor sees him in his vision of the events to come involving the Gauntlet, hence trusting him upon his "birth"
-His dialogue from beginning to end in the film is 100% Adam Warlock. In particular His position on life and death and the line "I simply am."
-His lifting of Mjolnir. Adam is a saint. He is more worthy than anyone else in the Marvel Universe to wield Mjolnir. He is literally the Avatar of Life created to oppose Thanos - The Avatar of Death.
-The butterfly he sees (which I thought was a dead giveaway) is probably the biggest nod by Whedon.