Because black's counterplay typically depends on the thematic break ...c5. This takes space in the Q-side, and often more importantly, allows the Queen access to that side as well. White's b2 pawn is a weakness in many d-pawn lines, once he develops his bishop and announces his plans on the kingside.
Trap in that pawn behind the knight, and the queen is stuck, the pawn is stuck, and white's chances on the kingside become brighter...black is going to find himself on the defensive, all while the knight is on a less than ideal defensive square.
Alternatively, black may be aiming for a Chigorin style defense, which is hardly unsound, but relies more on tactical themes than positional ones for black's counterplay. The downside there is that if white knows his way through the tactics, black struggles to find counterplay at all.
Why isn't the black knight on b8 usually developed onto c6 in the queen's pawn opening?