why not 5.d4!?

My guess would be because after exd4 cxd4 Bb4+ Bd2 Bxd2 Nbxd2 d5 white gets stuck with an isolated d pawn. 5.d3 is more solid. That's my totally uneducated opinion 😁

a video on this with Nc3: https://www.youtube.com/watch?v=LHvqrl5_JFA
The Bd2 line doesn't promise much for white: