Beta testers had noticed with patch 10 that the amount of space available in the shared stash was reduced, and Bashiok chimed in to address the issue. It appears that data storage concerns is the culprit behind this decision as Blizzard projected they could have issues with the influx of players at launch and items they store.
I can vouch for how Bashiok described the technical aspects of items as we’re on the tail-end of developing a pretty significant update to D3DB where we were knee deep in their item design. The only common variable on most magical/rare items is the base item. Affixes completely change how each item is stored and I don’t even need to do the math to know there are a lot of 0s in the total number of item combinations.
It’s not something that’s irreversible, the stash space will likely increase in retails patches as they improve their infrastructure.