If he truly was working on behalf of Russia, wouldn't he be gone by now?
Major figures within the Republican Party are likely compromised. Handle this wrong and the country could devolve into a minor civil war and that's not accounting for military splinters.
They can't just 'remove' Trump at the drop of a hat. There are few if any immediate legal mechanisms for the military and intelligence agencies to do so. You'd be talking about an asassination or coup by the military against the elected US President and it would be carried out in broad daylight.
The Republican Party itself is likely compromised, making escalation through official channels dangerous. It's also quite likely that what they had inially was circumstantial, without a smoking gun to leak or release that would force action in the public sphere. Given the high likelihood of that, once they started digging (post the firing of comey, mind) any smoking guns they may have found likely were buried within the sprawling depths of systemic corruption, necessitating time and meeting the legal standards necessary to uproot those webs then also become a priority. Unseating President shitbag world interrupt those goals.
And by letting Trump carry on as long as their hands are not forced, they can better uncover, define, entrap, and intimately expose the web while doing the least immediate damage to the core foundation of the United States as a democratic countryruled by laws and a system of governance once it all comes to a head.
Not too mention that they likely need the Republican Parties compromised elements marginalized further by 2020 to have the best conditions to proceed with what will define and damage the country for decades to come, if not longer.
