Erm, IP.Board doesn't have it either, and it's been a requested feature over there for quite a long time.
Give the XF lads a chance. I'm sure they want to offer most people most things, but there's simply not enough time in the day to program every feature into the first major update.
I'm sure they've got post edit history on their to-do list (oh, and it's not as easy a thing to add as you might think!).
Cheers,
Shaun