If you promote to a queen with 1. b8Q, black has:
1...Re2+ 2. Kd1 Rb2
attacking the queen and hinting at mate with Rb1++. If white takes the rook 3. Qxb2 it is stalemate.
Because of the mate threat white does not have any other good square for the queen either (no good check and no square that would defend b1).
Also you need to know that if the queen gets exchanged for the rook (e.g. after 3. Qb5+ Rxb5 4. Nxb5) , the endgame (king and tow knights vs king) is draw.
If you promote to a rook, black does not have this defense, because after 3. Rxb2 it would not be stalemate. The resulting endgame (King, rook and two knights vs king and rook should be a reasonably easy win.