Associate Game Director Explains Why Adding Stash Tabs Is Complicated - Icy Veins
www.icy-veins.com
Blizzard explain why adding more space to the stash isn't a trivial matter and how the stash and player interaction systems are connected.

Turns out they can’t easily add more stash tabs as the game pulls the entire stash for every person you encounter. This leads to memory and performance issues. They need to fix this before they can add more stash tabs.