Homeowners and landlords don't want more building for exactly the same reason: They own property there and consequently want it to become more scarce so the price of their asset goes up at the expense of tenants and new homeowners.

In general the homeowners are more numerous but the landlords are somewhat more organized.

The homeowners are also being taken for a ride, because less restrictive zoning reduces the value of units (since you can increase the supply of them) while increasing the value of each piece of land (since you can now build more units on it). And the homeowners (and small landlords) are typically the ones with a high ratio of land to units, but they get hornswoggled into demanding the thing that really benefits the corporate landlords who own the existing high density buildings.