The full length zipper makes it easier than high waist bib-shorts/longs... you're not embarrassed about your torso are you?
The only skinsuit I have ever tried had a half zip on the front, so getting into it was an achievement, getting out you needed to be Houdini. This may have been a very old skinsuit, I don't know, as someone lent it to me for my one and only TT effort, so I stand corrected if they are not like that now.
Because zip's are hard to undo
If they have full zips then I obviously would not be able to undo it anyway