Let's take Android, for example. Since Android started, it has grown at a very fast rate, with a new major release every year. That makes around 15 major versions out there in the market, out of which at least 6 versions are widely used in different parts of the world, depending on the region and smartphone availability.
These are only the major versions we are talking about; the minor versions are way too many to count.
Considering this situation, it is almost impossible for a quality assurance team to test the application and guarantee it will work on all the targeted devices out there.