People fogured out that there are better ways to abstract problems than what we thought 50 years ago. Hardware also has changed, now favouring a lot more concurrent programming on multiple threads, rather than efficient sequential execution.

In some cases languages have evolved but keep back compatibility, like C++, where you can use modern features like template genericity, metaprogramming, introspection, coroutines but still do OOP with virtual functions like in the early 2000s or pointer juggling and manual allocation like C.

This comes at a great cost of complexity, so people figured that limiting the scope of languages to only modern stuff frees up cognitive space and kept on adding languages.

Since software development is still a young science all this is still being figured out, so there are a LOT of new languages every year.