My very amateur analysis would be that if Nxb4, then c3 Nc6 Qa4 then attacks the Bishop, potentially providing support to the c5 pawn and removing the pin on the Knight, allowing it to attack e5 which is now undefended by Black's Knight.

My very amateur analysis would be that if Nxb4, then c3 Nc6 Qa4 then attacks the Bishop, potentially providing support to the c5 pawn and removing the pin on the f3 Knight, allowing it to attack e5 which is now undefended by Black's Knight due to the newly made pin by the queen.