There are several possible leading reasons:
1) Fundraising from the base. The Dems are significantly behind.
2) The next SCOTUS appointment. Knowing that the President is likely to be reelected, the Dems believe that if a President is under impeachment investigation, that can be used to prevent a nominee for SCOTUS
3) Damned if they do and Damned if they dont. Dems/swamp critters are losing their power to implement socialism under President Trump. Their loss of power has become so obvious that they no longer fear a loss during the election.
4) Got to show something. The Dems have not been able to show anything as a positive action to their base as a result of being elected. So they have to be able to say “look at what we did” about something.
I would add that the left isn’t just trying to get rid of Trump. It’s about delegitimizing the IG reports on FISA abuse which will take down the deep state.