Maybe because Pages does not have the Comics Sans font; at least not that I can find.
It probably replaced your Comic Sans with the nearest match, Chalkboard SE (or Marker Felt (thin)), which do not have a italics variants.
Hmm, I also notice that Noteworthy (next in line for Comic Sans likeness) also lacks italics.
Comics Sans used to be the default for the Notes app. It seems to have disappeared from iOS at the same time that the Notes app got updated, iOS 5.x. The Noteworthy font seems to have appeared at the same time, by not-so-strange coincidence.
No idea why, other than there is a lot of love-to-hate for the Comic Sans font in the typography world.