Because hoarding means missing out on a part of the game they want you to enjoy. And because if you let people get ridiculously huge inventories, then they'll hate having to manage those inventories.
Forcing people to do something they don't want to rarely makes them enjoy it. If anything, resentment is much more common.
But it sounds like the limitation is there to make some of the "inquisition perks" useful, that is, the inventory expansion ones. If that's indeed the reasoning behind it then they really should've known better -- they'd already made this mistake with the number of tactics slot in DAO tied to talent points you'd have to spend on them, and got enough backlash that they did away with it in DA2. So, in the end, no idea what they're thinking this time.