The reason is simple: suprnova is not a reliable pool..I've been mining various coins on it..it has so many 'issues'...so, people go to '4 miners' because it's a great stable pool...we pay a lot for our equipment, who wants to waste it hashing on an unproductive pool??
Interesting, what are the issues ? Actually I'm doing pools and crypto since 2012, I'm available 24x7, I even have a support chan in IRC or via Twitter.. I pay bounties for bugs, I pay bonuses, I even pay coins after rounds have completed if there were any issues.
I've never heard of any "issues" and the KR pool is running for quite a long time now.
Beside of that, I'm developing my stratums, by frontend and backend myself and I don't rely on any other developers who open source pool's backend software so I'm certainly aware what's currently going on in the background. I can see attacks on stratums, because I've developed stratums, I know how a fake share attacks works because I'm writing counter measures against it.. Or double share attacks, fake blocks etc.. Even IF such an attack sucessfully happens, I pay afterwards and reimburse the miners...
EDIT: Oh and btw there already was an "issue" which fast fixed quickly after it was reported and all users got a decent 20% bonus on their balances. I'm not doing this as my dayjob but I certainly serious in what I'm doing.