In point of fact, I rather think they will.  Just… not now.

Let us leave aside all discussion about whether Mr. Trump deserves impeachment.  There’s a widespread public belief that he does, and the present House was elected at least partly in order that he might be removed from office.  In the Mueller Report, the suggestion of obstruction is enough on which to base a charge — if not in court, then at least in Congress.  This plus a majority opposed is all that is necessary for him to be impeached, and any discussion of whether or not he deserves it is beside the point.

So given that impeachment once begun would be a foregone conclusion, why would the Democrats in general and Speaker Pelosi in specific not want to pursue it?  In part, the answer must include that the result would indeed be a foregone conclusion; while the House would certainly vote in favor, it’s the Senate that has the sole right of trial of impeachment.  And the present Senate will not vote to remove Trump from office; particularly as “…no Person shall be convicted without the Concurrence of two thirds of the Members present.”

The argument publicly expressed by Speaker Pelosi is that she doesn’t want to see Trump impeached; she wants him in prison.  And this may be the literal truth; while an impeachment would in no way eliminate the possibility of an eventual criminal trial — the Constitution is quite specific that double jeopardy does not apply — it would certainly be so public a spectacle that any criminal trial brought on the same grounds would be too prejudiced to continue in any jurisdiction in the United States.  All else aside, where could one possibly find jurors both disinterested enough to have not followed the Senate trial and morally competent to sit in judgment?

Political gamesmanship, however, likely has more to do with Pelosi’s decision.  There is, after all, an election fast approaching.  An impeachment proceeding at the proper moment could win the Democrats not only the presidency but also absolute control of the Senate.  If that should happen, the entire DNC agenda could be put in place at lightning speed — everything from universal healthcare to a gun ban.  Of course, if it’s publicly perceived that she’s doing so for political advantage, such a scheme is likely to backfire; as a result, I would expect her to step down from the Speaker’s chair or make some similar maneuver in advance.

“You want President Pence?  Because this is how you get President Pence.”

Consider this as well:  If Trump is impeached, Pence becomes president — and Mike Pence is wonderful insurance against impeachment.  Where Trump is generally seen as dishonest, unethical, and unworthy of trust, his Vice President suffers from none of those handicaps.  If anything, his sins are the opposite; he errs too far on the side of moral rectitude for the public’s taste.  While he might well not win a normal election, should Trump be removed from office, Pence could easily become the RNC’s white knight nominee even at the last minute.  If impeachment were to succeed earlier, that would give him more time to use his new office to campaign.

And so, from a strictly political perspective, there’s great risk to impeachment, even if it’s successful.  Likewise, we’ve shown that there exist several practical obstacles to the process, not least of which being that it’s unlikely to unseat him — and the failed attempt will make the Democrats look at best ineffective and at worst motivated solely by petty political rivalry.

It may happen that Pelosi has no choice.  She’s currently facing open revolt by over sixty members of her own caucus, and there’s no small number of Republican members of the House who would gladly cross the aisle to vote in favor.  If a strong enough movement were to gain power, she might even be stripped of her speakership; such a move is not without precedent.

Of course, Trump may yet resign from office, counting on a general pardon from Pence in exchange for his sacrifice.  But in any event, timing is essential, whether for Republican, Democrat, or Trump personally.  And now is not the right time — not for anyone.