Why is the engine evaluating against bringing more attackers in this position? [FEN "r2qk2r/pQpbbppp/n2p1n2/3Pp3/4P3/2N1BN2/PPP2PPP/R3KB1R w KQkq - 0 1"]


This position is evaluated at +6.4l but if I capture with the Bishop (Bxa6) it drops down to +0.8. 

Why does the evaluation drop when I make that move?

[Title "AKP2002-moisturiser, 600+5 Rated Rapid game, lichess.org, 4/21/2020"]
[FEN ""]

1. e4 e5 2. Nf3 Nc6 3. d4 d6 4. d5 Nb8 5. Nc3 Bg4 6. Be3 Nf6 7. Qd3 Be7 8. Qb5+ Bd7 9. Qxb7 Na6 10. Bxa6 Qb8 11. Qxb8+ Rxb8 12. O-O-O c6 13. Ng5 cxd5 14. exd5 Nxd5 15. Nxd5 Bf6 16. Nc7+ Kf8 17. Ne4 Bd8 18. Nd5 Bc6 19. Bxa7 Ra8 20. Bb8 Rxa6 21. Bxd6+ Ke8 22. Nc7+ Bxc7 23. Bxc7 Ke7 24. Bd6+ Ke6 25. Nc5+ Kf5 26. Nxa6 Rc8 27. Rhe1 Re8 28. Nc7 Rd8 1-0


Why isn’t capturing with the Bishop good, since it brings more attackers into play, and is hence better move?
Why is the engine evaluating against bringing more attackers in this
  position?


That's not what it is doing. It is evaluating against blundering a draw. After Rb8 your queen has nowhere to go apart from allowing a repetition. If you play Qxa6 you are just a piece and a pawn up for no compensation.

[FEN "r2qk2r/pQpbbppp/n2p1n2/3Pp3/4P3/2N1BN2/PPP2PPP/R3KB1R w KQkq - 0 1"]

1. Bxa6 (1. Qxa6) Rb8 2. Qxa7 Ra8 3. Qb7 Rb8=