The pile of mud has network effects. Even when you're starting from scratch, you're not really starting from scratch. The world is built around the things that are popular. Everything is better supported and better tested for those things. If you create a new language, it not only needs to be better, it needs to be so much better that it can overcome the advantages of incumbency. Which is made even harder when the advantageous characteristics of new languages also get bolted onto existing languages in a way that isn't optimal but is generally good enough that the difference ends up smaller than the incumbency advantage.
Which is why change happens very, very slowly. We're lucky to be essentially past the transition from Fortran and COBOL to C, Java and C++.