Aside from the dupe, the reason Stack Overflow requires a much higher rep for creating tags is, as the first and oldest site on the Stack Exchange network, with the most questions to date (Over 10 Million), allowing lower rep users to create tags can result in a burden to the site database. Smaller sites, will less questions, and as such, less tags, do not have this problem.
Generally speaking, each tier of site will have a different level of use/questions/users. Private betas, as the smallest user base, needs lower rep to privilege levels in order to allow users to access the moderation tools. A Public beta has more users, so a higher standard needs to be reach for the same quality of moderation. A graduated/Designed site out of beta will have even more, hence the higher rep requirements. It's done to keep disruptive privileges only in the hands of quality users.