same as the main thing that stops housing everywhere: the people who already own homes want the value of their own investments to continue rising uncontrollably, and building more houses has the potential to threaten that.

the stated reasons are usually that building more housing outside of existing urban areas is bad for the environment, and building more housing within existing urban areas is bad for the "neighbourhood character". but the reality is just that a housing shortage is good for a lot of people's bank accounts.