They talked about this in the most recent live letter. Sure money probably has something to do with it, but their explanation made sense.

Storing items increases the amount of data your player character takes up. By default you are given a ton of space, more than most MMOs (I mean 2 retainers might not seem like a lot, but isn't that more than 300 items AND the armory too). All of those items you store take up space and increases not only the data stored, but effects data access, every time you start flipping through all your retainers it taxes the server load a little. If they gave like just one more retainer to everyone, then suddenly item usage goes up like 50%. I don't think people pay for retainers as much as people think. I know the people (like me) that like to mess around with market board might have one or two extra.

How badly do you need that first coil armor piece that you haven't glamoured to anything for 15months anyway? I dumped the lot of the stuff I out-leveled except a few key things I liked.

I wouldn't mind more space personally, but I remember my WoW bank never had as much as this game does with no extra retainers.