why not Bb4? evan's gambit

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ryn45
1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Bxb4 5. c3 Bc5 6. O-O d6 7. d4 exd4 8. cxd4 in the game, Bb6 was played, why not Bb4 9. a3 Ba5 to prevent white Re1?
IMBacon22
Strangemover

White will play 9.d5 instead of a3. If the knight moves away Qa4+ wins the undefended bishop on b4.