I wonder how much the limited in game stash was a mixture of different things. Remember Classic only gave us 6x4. with LOD we had double that, but after playing D2R it seems far too small. D2R even with 4 empty tabs can fill up quite quickly depending how fussy you are what you keep.
1) Worried about storage space on Battlenet, bigger the space, the more data storage, the more electricity etc needed
2) Actual in game design of scarcity of space and needing to choose.
3) Perhaps overlooking how much people would want to hoard/share and how they would try to get around this. Luckily in SP we have these tools. You'd hear on Bnet about "perming games" and moving things. You wonder if they had either more space in game/easier ways to move things, they would have had to pay less in electricity for server costs if people could more easily do what they wanted?
4) I wonder how much it was about not wanting to design/implement some other way to move things between characters which we didn't get till D2R.
I'll tell you one thing at least I enjoy about their design philosphy these days. Its that they don't keep the special toys only in their online space, eg Pandemonium event etc which before D2R wasn't possible in forum accepted ways. The fact they have come up with things, and sometimes with a limited time delay they have come to Offline/Single Player is a massive PR win in my opinion.
I wonder if some of this was a ploy to get people to get excited about the Diablo Universe before the launch of Diablo4? Eg also see Diablo3 got its biggest and coolest patch a few months before D4 release.
One other thought I had is in some other place I saw someone sharing an opinion that they didn't think a certain streamer getting the grail was "legit". Their reasoning, in Single Player he had used the Players X command, which wasn't available on Bnet. To the commenter, the only acceptable grail was in Bnet on ladder. Basically we can never please all the people all the time. A place like this with common rules is good.