I think, and bear in mind that I am not sure, but I believe that the reason this is so is because Stack Overflow, given (1) its age and (2) its question volume, is much, much more likely than any other site to have all the tags it will need for some time. The only new ones that are likely to crop up are those of new technologies and those that are needed when tag cleanup occurs, and in both cases, a high rep user is likely to be involved. In contrast, SE 2.0 sites that graduated from Area 51 are smaller and newer than Stack Overflow and may not have all the niche tags that people might want to ask questions on, and so more people are allowed to create tags.