It is painfully apparent to anyone with a background in evolutionary biology. In biology, the only thing that matters is reproduction. Given a population with genetic variability and limited access to resources required for survival, individuals with variability that enhances their ability to procure resources will disproportionally have more offspring. Over time, individuals with this variation will become numerically predominant in the overall population.
So it is with COVID. The delta variant has characteristics that allow it to achieve greater virus load in its hosts. This greater load makes it massively more contagious: by as much as 1,000 times the contagiousness of the original strain. When individuals who can produce 1,000 more successful offspring appear in a population, they become predominant. The greater load has another effect. It makes this variant twice as likely to land its victim in the hospital to struggle for life.
Another fact is painfully evident to anyone with a background in evolutionary biology. As long as the world population is not predominantly vaccinated and, thus, offers breeding grounds for future COVID-19 strains, more virulent strains will arise.