And what exactly is the problem with having two 'zipping programs' installed? Does it harm your computer in any way?
I have Winzip, Winrar and 7zip installed here, not a problem at all and like DarkXCess said, WinRAR or 7zip can handle all other formats as well.
But to answer your question: Every addon/mod/mission creator has its own 'taste' and is using his favorite tool to create the archive. And since they don't get paid for their work, the customer (aka you) has to take what he got for free.
Just use WinRAR and stop the whining mkay?