User blog comment:Pamela.pajarillaga/Why?Can't I do it!?/@comment-75.36.133.223-20130816161217

Because it would complicate the program they would have to write. They have touted what Elizabeth does in BS:Infinite a some great achievement. There is a huge amount of scripting to have a NPC companion react correctly to so many general situations. Alot of the behaviors are stilll keyed to specific spaces in the terrain/levels (wich is alot of work to insert into levels)  and there are lots of special animations/voice spiels which had to be created and it gets repetitive quite fast, even so. A little Sister with  an even more limited repertoire would get aggrevating quite quickly (more effort to expand it sufficiently  than they wanted to do for that element of the game). So they limited it to the set down at the corpse draining and some comments to actions you do   while she on your back (there is a list of all the things LS say here on the wiki)