Blizzard did a similar thing with stash tabs in D3. They were really restrictive at launch, 4 or so. Just recently you can earn up to 10 with Season Rewards but its been like 3 years.

I know one of their responses was "the technology isn't there" and a lot of people balk and laugh and say Blizzard is dumb, but the truth is they are probably right.

Each player has 9 deck slots. If you add 9 more, then every has 18. If you have a database holding this information you literally just doubled the data entry for every user, which for Hearthstone is around 30-40 million users. Doubling that much space actually has consequences. Retrievals could be slow. Or the reindexing could take the game down for too long a time. Or they just don't physically have the space allocated at the moment. Its easier to add adjustments to space to meet current demand than it is to completely double space AND meet demand. All the same problems can apply to D3.

I don't know Blizzards back end, maybe the solutions were way more trivial than I think them to be. But It is nowhere near as easy as most people make it. It isn't just about adding UI or other components. There is a lot more we can't see that affects these decisions.