Same reason a lot of people use Word, revision tracking. Its the killer feature. Source code control, if it were for the normal person, could work but it would need some editor integrated tooling to show all the changes in place.
If you want to replace Word, make this your first and focused feature. The easy, no distraction writing folks are already numerous and successful for their niche.
I haven't seen a wiki with as good and editor as a standard word processor, also I'm not sure they are as distraction free and the current crop of distraction free apps like Scrivener and WriteRoom.
VoodooPad probably qualifies as a wiki / word processor fusion that works really well for writing.
If you want to replace Word, make this your first and focused feature. The easy, no distraction writing folks are already numerous and successful for their niche.