I’ll play devil’s advocate and throw out a few more possibilities.
Calling it “impeachment” focuses more public attention and allows the GOP to run their propaganda mill more efficiently . . .
Waiting until the Dems have the evidence lined up allows for some surprise. . .
There's a reason the GOP used the "I" word more often then the Dems in the Mueller hearings. The Dems used the word cautiously. . .
Compare caution to Political Theater and a Three Ring Circus (which the GOP wants) . . .
Not to mention, right now, Pelosi doesn’t have the votes.