because "some of the replies upset the OP".
Many posters spend time trying to help, of course some pile in and go OTT but that's par for the course on AIBU.
If threads stay they're useful for others in a similar situation who don't have the confidence to start their own.
The OP can stop reading the thread, or hide it. It's easy, I've done it myself when replies have wound me up.
Surely Mumsnet HQ could explain this to the poster rather than deleting everything and losing all the useful stuff?