Why did the protozoa not die out with the appearance of multicellular organisms?

All thanks to their adaptations to the surrounding world, rapid reproduction and small size. They inhabit almost the entire surface of our planet, and also live in water. For them, many unfavorable conditions are not terrible. Where protozoa died in the course of some causes, life is restored and comes from other places. Even a few surviving organisms can populate a large area in a short time. Therefore, many bacteria and protozoa live on our planet for millions of years and have not died in the course of natural selection.