Simple answer to topic: Not enough manpower. If a new game is developed from a big player, e.g. Blizzard, there will be around 200 persons working on it. Another 50 persons for marketing, controlling, comunity work...
Such a game costs millions of bucks and they are more and more reaching the production cost of big blockbuster movies.
Another dimension, I think! Do not know, how much people are involved in clod or bom, but I would giess 20 overall!
or something like that. You must think of the fact, that clod sells only a few thousand copies all over the world and some short time ago, only around 100 people worldwide were playing online. So, how to make money with such an estimation.... That is, why I have much respect for small producers and what they are able to produce with such limited capacities. They are really living a hard life!