MMORPGs: Why So Few Sandboxes?

Themepark-stlye MMORPGs seem to be everywhere these days, but a lot people would like to see another big-budget game take the sandbox approach. It has been a long wait, however, and there doesn't seem to be a whole lot on the horizon for sandbox fans. Are sandbox MMORPGs destined to be low-budget niche products? Here's my take on why themeparks have come to dominate the genre.