As with most providers - many run them but don't wish to.
It's incredibly hard to move away from those platforms.
WHMCS for example - there are absolutely alternatives, but the biggest fear has been opening a can of worms with migration related problems, especially when you rely on a lot of third-party hooks and addons. This is exactly why I said "if I were starting from scratch today, and had to choose between WHMCS or Clientexec, I'd go with Clientexec". Not impossible of course, but quite frankly there's better ways to spend time than dealing with such issues.
cPanel is so well liked in the industry and demanded by end-users. A lot of customers don't wish to use anything else. That is something I've seen first-hand, and just through reading what other hosting providers have had to say about feedback from their customers.
As for SolusVM, a plan is already in place to migrate to Virtualizor this year