Most End Users find it a very steep learning curve to move from cPanel to ISPConfig, and would rather pay the price difference than lose a whole bunch of unpaid time learning something new.
As for SysAdmins, I'm sure I'm not the only one who remember a new release of ISPconfig that had major problems, and was further compromised by the previous version's download having already been deleted from the ISPconfig site, and the ISPconfig support folks saying that it was impossible to bring it back thanks to the software they used.
All that said, with a few years having passed since then, ISPconfig was the best of the free control panels for my purposes (Development), but I finally decided to manual installation as I was needing to test beta versions of software, e.g. - PHP, and that wasn't very easy with ISPconfig.