Well, you can't just throw up condos all over the place and assume that alone fixes the problem. You have to build schools and other infrastructure changes, such as water and electricity to support the influx of a population.
Activism to push change requires a lot of energy and time. Most of the people who live here are busy with their jobs and families and do not have time to invest in it. So the activists who have the time to push usually want radical changes, such as 100% low income housing or homeless shelters. These are fine ideas, but for communities of upper middle class families, the change is scary and usually unwelcome. So then those families suddenly find the time to show up and protest and that's where you meet NIMBYs.
Change like this really requires the will of the local governments to make progress. Yet I think they get elected on other platforms such as, building better schools, or improving roads or downtown small businesses. YIMBY seems to be the first political wave of politicians actually running on a platform of more housing.