It is now very difficult getting a dog. Most people seem to want pedigree dogs, perhaps because they feel they need to have lots of documentation to show lineage and health checks.
It is more ethical to get a rescue dog but, having looked for one myself, there often seems to be issues such as: no visiting children, separation anxiety, destructive behaviour, reactive to other dogs, etc, etc.
The "puppy contract" provided by the RSPCA for buying a puppy/young dog is so complex with so many requirements that it is really quite off-putting.
Pet shops are not now allowed to sell dogs (or cats I think) and, on the face of it, that seems good. However, all these animals are now being sold on line. We bought our dear dog from a pet shop we had been to on many occasions. The animals were very well looked after and there were photos of various dogs in their new homes. We had a proper contract, showing date of birth, breed, etc, etc, and containing insurance provision for 2 months, and a vet appointment for health check and second vaccination. I am beginning to think that it was much safer to have a visible pet shop where animals were in view and which could be inspected if necessary, rather than using the internet where there are all sorts of scams involving puppy farms, often sited in remote places.
Sorry, I've drifted from the original title of the thread but I do think that the preponderance of pure bred dogs, is partly due to the much heavier marketisation of pets, and dogs in particular.