The benefits for making it larger don't outweigh the costs as listed above. We're barely getting along lag-wise as it is and I really don't want to introduce a heavier load, but that's not the only reason. I'd rather have planned map expansion as opposed to any bored schmoe being able to wander off and increase the map folder with literally thousands of extra files. Thousands of extra files to backup, to render in cartographer, to access from the disks, to keep in memory. Not to mention rendering new areas is prone to get the server into laggy "reboot me!" mode.
But sure, an increase in map size wouldn't be TERRIBLE -- if it were that important, I'd have limited it a long time ago. I'd certainly appreciate some restraint, however, and that people check the cartograph before deciding they need an entirely new area. We're already approaching the max map size of the much more densely populated PA server -- how much do we need? I see a ton of open space on our map!