One reason is that programming languages generally try to provide backwards compatibility and doing so limits their ability to fix design problems.

There's an interesting dynamic here; it is hard to see the design flaws of a language until it is widely used and common practices are established. Once that happens it is too late to change the design.

People occasionally try to fork languages to so they can break the backwards compatibility requirement, but I haven't seen many of those gain wide adoption (maybe you could say Python 3 is an example?). Instead there seems to be more success for languages that attempt to create a better version of an existing language but move far enough away from it to be an obviously different beast. I'm not sure why.