user21526: [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?

user4418: 
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=


21526: <pre><code>[FEN "r2qk2r/pQpbbppp/n2p1n2/3Pp3/4P3/2N1BN2/PPP2PPP/R3KB1R w KQkq - 0 1"]
</code></pre>

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

<p>Why does the evaluation drop when I make that move?</p>

<pre><code>[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
</code></pre>

<p>Why isn’t capturing with the Bishop good, since it brings more attackers into play, and is hence better move?</p>
 4418: <blockquote>
  <p>Why is the engine evaluating against bringing more attackers in this
  position?</p>
</blockquote>

<p>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.</p>

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

1. Bxa6 (1. Qxa6) Rb8 2. Qxa7 Ra8 3. Qb7 Rb8=
</code></pre>
