What Pelosi and Dems understand - when corrupt Senate acquittal is a foregone conclusion, the simple calculus is the longer this goes, the more perilous it gets for Trump/GOP Congress. It's a function of both more severe evidence coming forth after Trump's appeals are exhaustedin multiple cases (tax returns/DB financials/Mueller 6(e) materials/McGahn compelled testimony (multiple others then compelled to testify), and proximity of manifest GOP Congress' corrupt complicity to the 2020 election.
What Pelosi is doing now is saying the evidence we have today is damning and sufficient as a stand alone to impeach this motherfucker. But she's stating that without defining when a full case will be handed over to the Senate. If she came out and said we can't impeach "now" because of needing the evidence currently tied up in the courts, it would diminish the meaning, strength, and magnitude of evidence they already have - which is a shit ton. So her approach today is to stoke it, keep the pressure and momentum going forward, and merge it with the court outcomes into 2020 (that could end up being resolved a lot sooner than expected).