My understanding is that it happened at random, because the virus does not naturally optimise for severity, but for the spread rate. Which isn't too (or at all) linked with severity so as with each subsequent strain, spread rate always increased, severity of symptoms more of randomly changed. Because it's more likely for them to become lighter rather than more severe (there aren't too many "cracks" in our natural immune system to exploit, or we'd be extinct long ago, so a random change is more likely to be less than more severe), this is what mostly happened, with the exception of Delta strain.