Because breach was only good thanks to tejo +he got nerfed. Breach was never a top tier pick, but it was fine for a while thanks to cracked neon and cracked tejo. On his own he is shit unless you're very good with it and you have very good stuff prepped with it
Breach has been meta for a very long time and his nerf was a 0.2 buffer on one ability. Teams have been using him forever and most have good prep with him
I'm not saying he's optimal but the switch up is crazy sudden
he was never great (except when tejo was broken) and currently it’s just abundantly clear that either viper or deadlock is better on basically every map