Pelosi just does not want the Democrats to shoot their wad early and wind up with Trump acquitted by the Senate by election time. (Just like Bill Clinton!) That's a big part of Trump's actions and outrageous tweets. He wants to be impeached and acquitted. You could even say it is key to getting reelected. Nancy is not that stupid. She will move toward impeachment gradually, but she won't let it happen before election day.
The other thing that is happening is that there is a legal maneuver to get some Trump administration guys charged with perjury for taking phony arguments and making false statements to the Supreme Court in the census question case. The SC more or less reamed them a new one, so this could succeed. Judges do NOT like to be lied to, especially by government lawyers. Lies do have consequences, and Presidential pardons have no effect on contempt charges.
And finally, all these people who are ignoring congressional subpoenas are going to wind up in jail. They are out now but when this does get to court (before election day) it's going to look very bad with most of Trump's staff in orange jumpsuits. Remember there were 6 Watergate burglars but 40 of Nixon's guys ended up convicted.