If you're referring to relatively recent events (like the first 2 years of Biden's term) you need to look no further than Kyrsten Sienema and Joe Manchin. "Democrats" that opposed the agenda (and general Good Things(tm)) for personal greed and lobbying interests. Biden had a majority in the house, and a hostile supreme court, which we're going to be dealing with for a good long time that's to McConnell's fuckery. The defection of Sienema and Manchin made progress very difficult.

It's also harder to get the wider variety of interests in the big tent of left to go in the same direction. Kinda like herding cats.