There was a similar discussion a little while back that included an official response from one of the programmers, which explained that due to complications with how some other parts of the data storage (mailbox items if I remember correctly) were configured, it is actually impossible for them to add more inventory slots without a lot of reprogramming or possibly data loss. They worked around it as much as they could by expanding each ‘page’ of items from 25 to 35 but they cannot add more pages.

So, it looks like that’s where the saddlebag comes into it - they can’t make it part of the inventory but they can work around and give us this other thing that has to be handled separately but is better than nothing.