Politicians are beholden to the members of their district, not would-be members. Sure, more affordable housing would benefit many people, but it wouldn't benefit the members of the district, particularly the wealthy and powerful, since more affordable housing is very nearly equivalent to real estate values decreasing (at the very least, as perceived by voters). Sure, renters would in theory benefit, but real estate prices aren't rental rates, and the renters have to move to capture the improvement in pricing, often outside of the district.
Essentially, the status quo of real estate pricing always increasing in a district serves everyone within the district well enough. Any change to that is a massive financial risk to property owners, so it will be vigorously opposed.