They don’t implement it, especially not for money, because they think it would be an unfair move towards their modding community. It has nothing to do with engine limitations.
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I don’t get the problem. Except for a few sets or pieces like Silver Sentinel, epics are trash, and drop like candy anyways. Even more so when you do optimized campaign farming (which is outclassed by even aspirant crucible). Monster Infrequents don’t matter too much until the max level range, and when you get there, you can just make a character for them if you want, eliminating the need for storage.
The only reason to store epics is to do an emergency dismantling spree for materials if you ever find yourself short on them.
Anything sub-90 is a temporary novelty at best, playing with faction gear is perfectly workable too. Heck, some even just use the level 1 lokarr set for the whole run up to best in slot tier.
So all in all, thanks to the current balance, the stash tabs are more than enough.
Because they are making a expansion that they can add it in. And many of the community do not use any mods…
I TOTALLY agree we need more tabs!
Tetris-style… laughed my ass off… so so so bloody true!
I kinda like the tetris style, mostly because it makes the items look good in my inventory 
But of course it could be done differently (not necessarily better).
Like, reduce the height of bigger items like pants from 3 to 2, that would save a lot of space. Or just do it like in the Gothic games and make every item the same size.
These might be good ideas for GD2 when they make a new engine.
Right now I don’t think it’s worth the effort, like Zantai said UI stuff is not trivial and there won’t be any changes except the 2 new tabs. I think this should really suffice for the average player, and everyone else can use mods or finally start dismantling their pile of rusty epics 
“I don’t care if it’s impractical, Wendigomane Shorts are in right now.”