Board Thread:General Discussion/@comment-15494492-20160423211921/@comment-27701762-20160423235141

She might have felt she didn't need to resort to destroying the exit until Frisk decides to leave. Given how infrequently the humans fall into the Underground, each time she meets and takes care of them she might hope that she can encourage them to stay with her without needing to force them to stay. Once Frisk falls down and starts asking about how to leave, she then realizes that she has no choice (since it's the last soul Asgore needs).

In addition, sometime before Frisk falls down she befriends Sans and exchange jokes, and destroying the exit would presumably prevent them from communicating. So there's some incentive not to destroy the exit unless absolutely necessary.