Three things, off the top of my head:
1) It's legitimately illegal 2020 election interference, so it kills the "wait until the ballot box" argument against impeachment.
2) It's very much stepping on the Congress' toes, insofar as the White House has no authority to withhold a whistleblower report.
3) It's a simple, cut and dry narrative of an illegal act, with confessions in the media from Trump and Giuliani.
to add onto point 2, it's also Congress' job to authorize foreign aid. Trump used some bullshit to delay the aid to Ukraine. So he stepped on their toes in a few ways.