"We assess..."
That is a phrase that security services use when they're just making it up without sufficient evidence. It's an opinion, nothing more. The same term was used for the reports on Iraq's WMD.

They assume that Putin ordered the hacking, as opposed to the Russian intelligence services doing it as a matter of course. They assume that the intent was to influence the presidential election. They assume that the goal was to undermine public faith in the US electoral system and Hillary Clinton. They have piled assumption on top of another layer of assumption, piled on still another layer of assumption beneath that. As for the Russians preferring Trump to Clinton, they're perfectly free to do so. All countries have a preference concerning the foreign governments they deal with.

The heads of every single US security and intelligence agency are Barrack Obama appointees. They in turn choose the people below them. You're hearing what they want you to hear, and not one fact more.