Because nobody is running tests for NT4. It's not that newer Perl releases won't run on NT4, but there is nobody testing the compilation of Perl on NT4, and thus it falls out of the observable universe of Perl.

For Win9x, the situation is even worse, not only is there nobody testing, but even building on Win9x is impossible due to command.com being the shell. Dropping Win9x support has the actual benefit of being able to drop some dynamic loading of API functions and thus simplifying some conditional code.