I think it is because of the experience of the first impeachment. Many of those closest to Trump refused to testify then, to make them do so would have involved a long drawn out legal process.Then why are they not calling these people as witnesses?
White House Officials Fail to Appear for Impeachment Inquiry Testimony
Four administration officials whose testimony House impeachment investigators had scheduled for Monday didn’t appear, amid a legal standoff between the White House and Congress over whether they can be compelled to testify.
I don’t know if McCarthy would be a legally in a different position as he wasn’t part of the White House but since Kevin went groveling to Mar A Lago recently as well as not voting to impeach he’s likely to throw up every single roadblock he could, honestly wouldn’t rule out him refusing to speak if they did get him on the stand.
Assume it is a balance - how long do you tilt at windmills trying to get GOP to admit what Trump did versus focus on other urgent business to get Biden appointments in place, COVID measures passed and other policy goals.