Why Did The Russians Share Their Secrets With Us?
Short answer is they probably pulled a scam that either fooled US authorities (or they are pretending to be fooled.)
The US intercepted a Russian diplomatic message that indicated that Jared Kushner wanted a secret communication system within the Russian Embassy for messages with Moscow. One could think that the Russians made it up, although attacking a friendly administration may backfire. That explanation is doubtful since Jared Kushner claimed that he did to talk about things like Syria. Syria sounds like bullshit, but okay.
There are only two logical explanations left. The first is that no collusion happened, and Russia blew the chance of a life time to potentially co-opt a presidential son in law and senior White House official by letting U.S. Intelligence intercept their message. If they couldn't transmit a secure message, they could've just had someone fly to Russia with that message. This would be a record breaking intelligence fuck-up and is therefore incredibly unlikely.
The second explanation is that there was collusion and that there was and probably still is a secret communication channel to Russia. What almost certainly really happened is that they already had a secret communication system and sent out the report that was intercepted by US Intelligence to make it seem like they didn’t have one yet when they already did. It’s disinformation and almost as damning as any confession.
The Trump team always denies bad stories, but admitted to this one. Maybe it's because the truth is far worse. Ask Trump’s team about this in interviews and see what they say. They don’t have a good answer.