I'd argue exactly the opposite, the reason there are so many is that it's really hard to write a good data visualization framework. Data visualization encompasses a vast range of different tasks that are often only loosely related, and it's extremely difficult (if not impossible) to write one library that serves all uses well. Add on top of that the fact that you have different output types - web, interactive analysis, print etc and you end up with a situation where you get lots of different libraries optimized for different use cases.

And that's not even counting differences of opinion on aesthetics...