As far as I understand it, IPAs don't require specialty equipment and they don't take as long to brew, so you get quicker turnaround and need less storage space. When you are running a large beer company out of warehouses on the edge of town, storage isn't as much of an issue. But when your beer is being made at a downtown microbrewery, there's a huge cost to that extra storage space. If your beer takes two weeks to brew instead of six, you can produce 3X the beer in the same space.