Past PMs were resigning over losing more trivial votes by more trivial margins, and this is definitively not a trivial matter.
Normally it'd be a fatal blow, but I guess no Tory wants to challenge May because of the mess to inherit or simply because there's no stalking horse to trigger the race ("a regicide cannot inherit the throne" mentality).
Strictly speaking they cannot challenge her, she's safe from an official vote of confidence as party leader until December 2019 and leadership challenges no longer exist - thus no need for a stalking horse -.
I miss the old times before that vote of confidence to last for months/fixed term garbage.