Making a space marine is a very costly thing to do and there are limited ressources for that. Space Marines are counted by the thousands, generic human soldiers and citizens are counted by the billions and trillions.
Abhumans, humans with certain mutations, like Squats or Ogryn are part of that group. They simple are the way they are due to adaptation to their environment and nobody bothered to change them.
Besides, in the world of WH40k, there is no technological advancement anymore. They might know how to make space marines, but comparable but different procedures are impossible. They have no scientists making new stuff anymore, only scholars and archeologists unearthing knowledge from 10,000 years ago.
