With the pawn on the seventh and its king beside it, the rook must stay on the back rank in front of the pawn; any other rook move allows promotion with check or a tempo-winning king move.
White has only a rook and must find a way to stop the b-pawn, which is already on the second rank and supported by the king on b3. The white king on e1 is close, but Black threatens both ...b1=Q+ and ...Ka2 followed by promotion.