They needed to score two runs and not just one.
Probability of the home team scoring two runs in the bottom of the 9th, down two runs, with runners at 1B and 3B, and 0 outs: 17.5%
Probability of the hone team scoring two runs in the bottom of the 9th, down two runs, with runners at 2B and 3B, and 1 out: 23.13%
https://gregstoll.com/~gregstoll/baseball/stats.html#H.-2.9.1.7.0.0