Most importantly, there is a very strong motive for Russia specifically to do so. And the rhetoric of the Trump administration, both before and after the election, bears it out.
Most tellingly, there was exactly one thing that Trump asked to change in the then-preliminary GOP platform when he became the party's candidate - and that was the stance on Ukraine. The original document included language such as "lethal defensive weapons" - which was what Republicans in the Congress were demanding from Obama ever since the occupation of Crimea - and then it became the much more vague "appropriate assistance".
To Russia, this is crucial, because its ability to wage successful "undercover war" in Ukraine hinges significantly on its technological superiority. For example, by supplying the local militias with new and shiny tanks with modern reactive armor (and some "vacationing" drivers and gunners), they significantly diminish AT capabilities of Ukrainian infantry; and in a war where both sides are unable or unwilling to use the air force, armor is the next best thing when it comes to close infantry support. But if Ukrainian troops were to get, say, Javelins (the single most asked thing by them to date), that would change matters a lot.
And it's not that Russia couldn't crush Ukrainian army, such as it is, in a head-on confrontation, Javelins or not. It's just that it would be too overt for plausible deniability.
Their job is to find it out who did this, informing the US population is not part of their job and quite often is counter productive. Don't trust any of them, your interests are more similar to a Russian civilian than any of the agencies.
I'll trust the CIA over the FSB/GRU any day of the week.