Actually, that is not the big problem. I mean, it's easy enough to just exclude the optional characters from the FMVs (as happened with Vincent and Yuffie in VII).
No, the real problem is the rise in voice acting. The resources and effort it takes to make a new character with full voice acting (and the more detailed graphics) is considered not worth it if the character is not going to be seen by most players.
Personally, I think it's a silly reason, since all the players who love the game would get the unlockable characters, but this has actually been a confirmed by BioWare as the reason why certain characters from ME1 only got cameo parts in ME2. (SPOILER)The characters could die, so they won't be accessible to everyone, so they get cut to cameos. Given that we've already seen Square-Enix's position on things that require too many resources with XIII's lack of towns, I think similar reasoning will hold true for them.