The cap is partially Blizzard's way of throttling item inflation. The larger the cap, the more that the highest-performing players dictate the values in the AH.
If you're finding items fast enough that nothing you have in the AH is under 20 mil, then you're probably vendoring a ton of items that could actually sell for several million, simply because you can't sell. Or, you might have a second or third account, or be using lots of mules, etc.
If the cap is raised, your top items sell faster (if it's in your stash, there's no chance it will sell). As a result of this, you sell further down your list, meaning that you put even more downward pressure on AH prices.
The cap doesn't have much of an effect on players who are slower or play less, because maybe you're only finding 2-3 items/day that you think are worth selling.
I think the cap is fine. I think it would be interesting to make it a bit more flexible, though I'm not sure what the right system would be. Perhaps an "overflow listings" tab where you can list another 5-10 things but the cut is even worse so you only get 75% of the proceeds.