The available range of frequencies is smaller in 2.4 GHz, so fewer networks can run in parallel without interfering with each other. Even in early standards with 20 MHz channels, you can only fit 3 networks in 2.4, vs in 5 it's more than 10 (exact number depends on region and other special cases). 2.4 Ghz is also somewhat better at getting through walls, which one the hand is useful, but on the other means in a dense city networks from further away can interfere.