Most of the voting population are homeowners who have a vested financial interest in the cost of housing always going up.
So anything that makes housing cheaper is politically difficult.
Now, I personally think that A) a static neighborhood is a slowly dying neighborhood and B) when almost every neighborhood acts this way the cumulative negative effects on society quickly become profound, but I can at least understand why, and can see the impulse as not an entirely selfish one. It's just naive and short sighted.
But a huge chunk of them don't think that way and they want to maintain the "character of the neighborhood." I've seen this first hand multiple times. My own grandparents once tried to argue with me against any sort of expansion because it would mean some people would be effected and have to change their lifestyle or pay for changes as a result of, for example, expanding city sewer services.