New housing lowers property demand.

Plus, people usually oppose nuclear which is an expensive but stable renewable option.

Lefties want new houses ? Thats a strange claim to make. The most left wing states in the US are the ones that build the least new buildings.

Rich corporate liberals are into trans rights, keeping abortion, etc. but they arent keen on new housing either. It puts a dent in their property portfolios.

Some people call these people "the left", I suppose, in which case yea they're not much keener on new housing than the rich corporate conservatives.

Theres also an element by which it is driven by high prices and loss aversion. A new homeowner with a $300k house and a 95% mortgage is gonna locally fight anything that drives up supply and drives them into negative equity even if their sympathies do trend left. Nobody fights new development as hard when prices are lower, which is probably partly why NY's strictest rent control (which drives down prices) coincided with the fastest building in the 60s.

They want new housing but often built by the government instead of evil property developers.

And new housing, .. but not built in their neighborhoods. Oh, and don't build over farmland or undeveloped green spaces.

Without being snarky, the problem here is that there is a tension between what people believe in general, and what they believe about their situations specifically, especially when they have an incentive to feel the opposite way about it. For example, people may know (or believe) that building duplexes and apartments in their neighborhoods will reduce their property values, or "ruin the character of the neighborhood" and so oppose them.

Private property developers simply dont want to build affordable housing. The margins are just lower than they are for luxury housing. Who would choose to make 4% profit when you could make 20?

They do lobby hard to remove building regulations in the name of building more affordable housing but they never actually build it unless local government actively forces them to with some of those regulations they hate, at which point they build the bare minimum and install a poor door.

They're always going to build the highest margin product for which there is demand. You're not going to get cheap housing until there is abundance. It's simple supply and demand.