The left, which is always in favor of increased federal spending, presumably wants the bipartisan infrastructure bill to pass. They want the human capital bill to pass even more, however, so they are holding the infrastructure bill hostage. It is a very Trumpian tactic.
The threat to torpedo the infrastructure bill is simply not credible, unless you assume that the real objective of the left is to discredit the Democratic Party, prove that it is unfit for office, and put Trump and McConnell back in charge of the country. Furthermore, voting no will only cause the moderates to dig in even further on the human capital bill. A no vote is actually the opposite of the use of leverage.
If I’m Pelosi, I would bring the infrastructure bill to a vote ASAP and see just how suicidal the left really is. My guess is that they will cave after the time for loud posturing expires.