Pelosi seems to understand something that Peter Rodino (and to a lesser extent, Tip O'Neill) understood in 1974: that the public has to be brought on board for any potential impeachment to succeed. The vast majority of the public hasn't read the Mueller report, and investigations are the tool to inform those who need to be exposed to the evidence.
Rodino's methodical approach - which was opposed by some on the Judiciary Committee staff, also allowed him to keep the Southern Democrats on board, which in turn allowed him to obtain enough Republican votes to squelch accusations of partisanship. It's debatable whether anything near that level of bipartisanship is achievable today, but a rush to impeachment would rule it out completely.
Also, as has been pointed out, at the moment she's dealing from strength. As the legal battle continues the Administration's position gets weaker and weaker, and its shortcomings become more and more evident; to abandon this approach in favor of a political fight, would only squander this advantage.