May's performance as PM has hardly been a shining example of how to lead, but it could be a lot worse. To hold a contest now would be as big a gamble as holding an election and then failing to campaign.
I don't have an axe to grind really, but I think that 'better the devil you know' if one wishes to minimalise risk. She, at least, appears to now accept Brexit, and just needs keeping an eye on in case she starts offering even more concessions to an intransigent EU negotiation team and throwing more of our money at them.
Brexit is a major part of a PM's leadership responsibility at the moment. Change the leader and one risks the country being sold out completely by a remainer PM, and that would be a disaster, and would ensure the issue would need revisiting, with all the resultant chaos, for years and/or decades to come.