Mostly it’s because, information wise, it’s almost nearly “free” to take a design and duplicate it…bilateral symmetry is natures version of copy/paste.

With that in mind, it’s likely that non-‘bilaterally symmetrical’ organisms relatively regularly spontaneously develop it due to random mutation. Just like we often randomly find people with extra fingers or only one set of organs, over millions of generations, bilateral symmetry will naturally just happen. The difference being, extra fingers or ‘more than two’ organs rarely offer any evolutionary advantage, especially in already complex forms.

Millions of years ago, however, very simple organisms suddenly having two brain lobes, two eyes, twice as many fins, two gills, etc….for free (informationally) and at only a relatively higher cost energy-wise could have found itself at a distinct advantage. If you can both run from predators and towards food twice as fast, and the energy cost isn’t twice as much, you’re suddenly the two legged guy at the ass kicking contest in a parade full of one legged people.