1) Because with 3... Bb4 Black develops very quickly, which is comfortable (Black has won many model games in the 4. Qc2 variation by using this early lead in development).
2) 3... d5 is somewhat rigid and defines the structure very early; after 3... Bb4 Black still has the choice about what to do with the central pawns.
Flexible and active it's no wonder the Nimzo is one of the strongest defences against 1. d4.
Why do people with with the Black pieces like playing this opening so much against 1.d4 ?
Why is 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 more popular response than
1.d4 Nf6 2.c4 e6 3.Nc3 d5