"Mine workers were mostly men (42/44); median age was 53.3 years. Eighteen of the workers had risk factors for severe COVID-19: high blood pressure (11/44), diabetes mellitus (4/44), or obesity (4/44)."

"The overall attack rate was 54.5% (24/44); 87% were symptomatic, 65% with fever, and 22.6% with dyspnea. No clinically severe COVID-19 (1) was observed, and no patient was hospitalized."

This make sense. If more severe variants are more damaging (i.e. more severe symptoms, death, etc.) they are less likely to persist. The less harmful variants are more likely to carry on.

At the extrene, for example, look at Ebola. It's quick and it's deadly. Consequently, it burns out. On the other hand, a longer asymptomatic incubation period - long enough to get on a plane - and Ebola is going to be big trouble.