There are several aspects to this. The first is that, as far as I can tell, there is no clean separation between code and templates in the perlmonks code, so modernizing the generated HTML involves far more than replacing a few templates.
The second is that among the many problems that perlmonks has (regular performance issues, broken Unicode support (for example in code tags), spam, ...), the design is one of the lesser worries.
And the third (please correct me if I'm wrong here) is that perlmonks doesn't really have any active developers. There are some pmdevs who occasionally tweak something here and there, but I don't have the impression that anybody is willing to do some major changes. Of course the lack of a test system would make such changes a major PITA.