Per Obama's spokesperson Josh Earnest: “Given that the president had endorsed a candidate in the presidential race, he believed it was important for the intelligence community to make this announcement,”
Also, due to the political nature of the leak, Earnest has stated: “it would’ve been inappropriate for White House figures — including the president of the United States — to be rushing the intelligence community to expedite their analysis of the situation.”
Bottom line is Obama didn't want to make it look like a political anti-Trump witch. He (falsely) believed that if the info came from the intelligence agencies themselves through their own process, instead of through him, that the info would be believed. But as Chris has pointed out...willfully ignorant Trump supporters have sided with Putin's version of the story over our intelligence community.