As compared to where? European languages are no more diversified than African or Asian languages.

Are you comparing it to the Americas where 90%+ of the population speaks either Spanish or English?

The question you asked should be reversed, actually: Why are languages in the Americas so non diversified?

Well, it's because of the European colonization. Before they showed up and forced everyone to speak Spanish (or English or French), the Native Americans were also quite linguistically diverse.

So you had a great homogenizing force: empire-building Europeans showed up and got everyone speaking relatively few languages on a whole continent.

Since that time, there hasn't been enough time for signifiant linguistic divergence to occur. That, and globalizing forces whereby speakers of a language in a region are no longer as isolated as they once were in the past, has kept the pace of linguistic divergence slower than it otherwise would be.

All that being said, the USA for example has several regional dialects (consider "California" English versus "Boston" versus the way a speaker in the deep south speaks). American English in general has diverged from British English in lots of ways, but not to the point of no longer being mutually intelligible (although take a speaker of English from Manchester and one from the the country in Arkansas and see how much they can understand each other -- it might take them a few minutes to find common ground).