Programming Languages seem to evolve over time. No one has designed a proper programming language once and then said "okay, it is done!". The CeeLanguage is sort of done and finished, but not really since there was C++ and ObjectiveCee. Pascal was finished once upon a time, but then had to evolve into Apple Pascal and Delphi. PHP 3 was a success but then evolved into PHP 4 and PHP 5.
If there was a really good language designed it wouldn't need to continually evolve. Evolving programming languages are proof that humans suck at design - we are poor engineers, poor mathematicians, poor architects. Or possibly design sucks, and evolution is better.
Or maybe the conditions have changed over time. A number of trade offs that were good ideas years ago aren't such good ideas today.
I think a combination of evolution and design is best - pure evolution is way too slow, just look at how long it took for humans to evolve to where we were; millions of years - that is too long to wait for a good programming language!
Possible Reasons for Change: