We had much more variety of OS in the past, but over time people flock to a subset for various reasons and the less successful ones die out.

After a point the waterline of what the bare minimum an OS should do rises with these remaining successful OSes, which raises the bar of entry eventually to the point where a hobbyist developer is very unlikely to be able to create something close to competitive. (Well except in niche areas not served by the big OSes, which is not common given Linux)