The Dark Matter Halo Gets Too Big

As galaxies get older, they pull in more of the stuff around them. They get bigger and heavier, which makes em more gravitationally powerful. Gravity sucks in the gas may be too fast, and so when it sucks it in, it's too hot, and it can't form any more stars. All this just comes together to give you a picture of star formation in galaxies as sort of fragile and special,. like you need these special conditions.