when fixed costs are high you want to invest more to raise the final price. The lower the fixed costs the less sense it makes to build the most expensive thing you can.

there are only so many people looking for those units. As more units are built the competition to sell those units will heat up. Either prices drop on those units, or theyre replaced by units that are cheaper to build. Right now they can sell all the luxury units they build so no reason to build anything else. So removing the barriers doesnt itself make luxury less appealing for developers, but it increases the number of units built and that makes luxury less appealing for developers. It will still make sense to build luxury, but it wont make sense to build only luxury.

There are other reasons too. Personally I would not mind living in communist style blocks and I know some others agree. If it was legal to build those they might be created because you can fit so many more units on a plot compared to luxury housing.