Because TIOBE's methodology is terrible, and always has been.
https://www.tiobe.com/tiobe-index/programminglanguages_defin...
It's literally based on the (estimated) number of search engine hits for e.g. "Python programming" on Google and a handful of site-specific search engines (e.g. Amazon, eBay, Walmart, microsoft.com, etc), with some manually applied tweaks for languages with easily confusable names. Many of the fluctuations in rankings - like the big dip in the ranking for C between 2015 and 2018 - probably have more to do with changes in search engine algorithms than any real change in popularity.
This is usually considered a better ranking. This has Rust at #20.
Different rankings measure different things, so it depends on what and how people want to assess languages and usage.