Democrats elected in 2018 take office in 2019. Democrats will already be running for President at this point, likely including many Senators.
They also won't have the seats to convict. So they spend a year or more bringing the charges and doing the trial and we're already into 2020 at minimum. The GOP could have dropped Trump!
And if you succeed, then you have President Pelosi, do your candidates all stand aside so she can run for re-election?
It's a logistical nightmare. Instead you just keep the cloud of scandal active.
I've seen some evidence (but its been years) that there was significant disapproval of both Clinton's actions and Republicans prosecuting his impeachment, and it got taken out on Gore. (Which led to Lieberman.)
This is basically why they didn't impeach after 2006. 2008 accomplished the same thing and was seen as far more legitimate because the "people decided" rather than months of people hearing all the impeachment garbage they hate because they hate scandal just generally more than specifically.