I suspect Canon thought, correctly, that the typical user of these cameras is a consumer whose best choice is using 30p (29.98) in the U.S and other NTSC countries and 25p in PAL countries because that is what their televisions use and what most basic video editors for consumers use. Thus they were trying to simplify the choice that gives the majority of the buyers of those products the best user experience.
By adding 24p as a firmware update, they can have their cake and eat it too. The average home video shooter isn't likely to do a firmware update after the purchase. The rare wanna-be filmmaker that thinks even the cheaper models should offer everything the higher models do can now STFU about no 24p on a camera they never, ever had any intention of buying even if it does offer 24p.