I think what was previously mentioned about Minecraft to be true, had the educational potential had not been realised.
Where Blockland and Minecraft differ was through their community; one had a dedicated team of moderators that would tackle the bad stuff, and the other just had three, one was pretty power hungry and a textbook national socialist-mod - eventually getting the sack, one was very polite but still doesn't really do enough, and one did all of the work but only comes around to sweep up the rubbish every so often but significantly less than he should. It would be better to sell a product with a good community than have another with a really bad community. Imagine the negative press Blockland would get if it went down the same path as Minecraft and got picked up for the potential it had.