The TIOBE index is effectively just based on the number of estimated hits for "$LANGUAGE programming" on Google. That's it. It's not based on actual usage, or interest, and certainly not importance; it's just counting search results.

(They actually search on a few sites other than Google, but they're bizarrely chosen -- they include search hits on Walmart and Etsy, for example. And the way they weight search results effectively means that the wildly inflated Google search result counts dominate the ranking.)