This observation by Politico spells it out pretty well
This piece is called "How Trump has handcuffed Washington" notes this salient fact:
DEMOCRATS decline to impeach the president because, in part, they’re worried about a sliver of members of Congress who have slivers of their constituencies who might or might not be offended by impeachment proceedings.
We are looking at a situation in which certain freshman Democrats are being encouraged by leadership to stand their ground against impeachment no matter what is revealed about the president because as that arch quote points out "a sliver" of their constituencies might (or might not) be offended. This is distorting the process.
Last night we learned that the president is trying to sabotage the 2020 election by extorting a foreign power to smear one of his rivals. There's nothing new about that. He was happy as a lark when Russia did it in 2016 and has suffered no consequences for doing it. Of course he's out there soliciting similar help from friends and foes alike. He considers this normal politics. He's said as much.
Some people might think that using millions of taxpayer dollars to extort foreign governments to do his campaign's dirty work is just a teensy bit beyond the pale and maybe worth taking a risky vote on in those districts with a sliver of possible Trump voters. After all, if the Republicans can use the power of the presidency to steal elections they probably won't be able to keep their seats anyway. It seems like a risk worth taking.