The only thing I can think of is perhaps saving time and resources, but if you think about it for more than a minute you realize it doesn't change anything in the amount of work they have to do.
Because even if they lock us out of some content by removing certain characters from one playthrough, the fact we can choose our three out of all the companions means they have to account for any choice anyway and it's still the same amount of work. So I really have no idea what the hell are they thinking
