The reason in part is because of the lack of unit tests of regrtest (as commenters above have noted).  By preserving the getopt interface, we can keep almost all of the untested code as is.

You should view the patch as a first step in refactoring to use argparse.  We can remove the conversion code and the main for loop in later steps.

Note to committers: I was meaning to rename regrlib.py when I was working on this.  Until regrtest-related code is in its own subpackage, I think the file name should begin with "regrtest" -- perhaps regrtester.py or regrtestlib.py.