It is true that there are many more ".mobi" file then ePUB.
But truth is that as long as you have the Kindle file ".mobi", there are numerous free applications that can translate it to ePUB - Thus allow you to get the book on the iBOOK as well
One such free service you get right here:
Convert Files - free online file converter and YouTube video downloader.Convert videos, audio files, documents and ebooks.
So all my mobi books are already in ePUB as well .