Because developers want to target the whole Linux desktop segment without having to reproduce the packaging process several times. Has nothing to do with whether the format is better or worse as long as it is consistent. Fortunately, gems like FPM have attempted to address this issue.
One of the main advantages of Linux and all the different distributions is the freedom the users have to have a system that matches his or her needs. This has been the case for decades. There has never been a single packaging solution and I see no need for one. Of course packagers want their lives to be easier, but the request to target the whole Linux desktop segment without reproducing the package process is pretty unreasonable given that different users simply want different packaging systems. Developers should weigh the advantages/disadvantages of supporting (say) Ubuntu or CentOS just as they do for Mac and Windows.
