All signs point to Russia; no signs (currently) point away from Russia; the public and private sector are in agreement; Obama is ballsy enough to call them out on it to their face; Russia has plenty of motive and more than enough talent to do it; it's probably Russia.
I'm not saying I trust the current administration or the intelligence community at all. I also wouldn't put it past Clapper to lie to other countries about various matters (just as he repeatedly lied and probably will continue to lie to the American public and Congress). Their job is to lie. I'm sure they lie about a lot of things.
But it would be dumb of them to make this particular accusation formal without pretty good proof, and all other independent analyses published support the accusation.
When they're willing to lie to our face under oath, trying to sell us secret evidence isn't going to buy much.