there is a saying. "A knight on the rim grows dim". Knights are stronger in the centre of the board. This is because the have more movement. (8 squares compared to 2 or 3 when it is on the edge). So moving the knight back would worsen it's position.
Why 27: ...Qxb8?
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I feel like the above is an incomplete answer, since its really a choice of which piece to undevelop. The knight on b8 is totally useless and will need to spend a move to move back into the action. Whereas on b8 the queen is still on that diagonal which might be useful in restraining f4 and attacking the dpawn if it advances. Also, I'm also not totally sure the queen wants to be on d6 with ne4 coming? So its more logical to decentralize the queen.
This game is Evans-Sandrin 1948 U.S. Championship. Why does Black move 27: ...Qxb8, instead of Nxb8?