Kennedy is the reason the SC won’t take 2A cases cause neither side is sure which way he’ll vote. Would rather lose a solid leftist but if we can replace him with a Scalia it’s ok.
There are two likely factors, IMO.
- Kennedy is a moderate. He was the swing vote in Heller, but it may be possible that he isn't a big fan of extending 2A further at this point. I don't think he was gung-ho for Heller; word is that Scalia had to add all of the language about not casting doubt upon "longstanding prohibitions" in order to attract his support. He leans pro-freedom but he is at his core pragmatic and averse to ideological rulings.
- Roberts is an institutionalist. He may be reluctant to take on 2A cases out of worry that extending Heller would further politicize the judiciary and harm its influence in the long run. There is strong evidence this is why he upheld the Obamacare mandate under the Congress's tax power.
Most likely, both factors are at work - Kennedy is squishy on how he would rule, and Roberts is squishy on whether he wants the court to take the cases right now. So it makes it an easy call for Roberts to vote against taking the cases up - avoid the controversy, avoid the potential bad precedential outcome from Kennedy going rogue.
If you replace Kennedy with a solid 2A advocate, you have four guaranteed votes to take up cert. And once they're forced to take up the case, it's hard to imagine Roberts not being on the right side.
Short version - Kennedy retiring could be huge for gun rights.