As the other answer points out 5. e3 is more flexible but there is a very concrete reason too; After 5. Nf3, Black can play h6 6. Bh4 dxc4 7. e3 c5 and now we are heading to an isolani type of position that tends to drawish positions and may not suit everybody's style:
[Title ""]
[fen ""]
[Startply "21"]
1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Be7 5. Nf3 h6 6. Bh4 dxc4 7. e3 c5 8. Bxc4 cxd4 9. exd4 (9. Nxd4 Bd7 10. O-O Nc6 11. Nf3 O-O 12. Rc1 Nh5 13. Bxe7 Qxe7) 9... Nc6 10. O-O O-O 11. a3 b6 12. Qd3 Bb7 13. Rad1 Nd5
Moreover, if Black instead of 6... dxc4 plays 6... 0-0, the main move for White is e3. This means that by altering the order (first Nf3 and then e3) White has to study the shown line above, whereas in 5. e3 Black has to study additional lines like 5... 0-0 6. Rc1 or 6. cxd5, so 5. e3 is a more pragmatic choice.