The popularity of salon is uncontested - if you're new to grooming, you're most likely going to go to a salon as most of your friends and family probably take it to a salon.
Its the most tried and true businesses - they have been around for decades and people grew up knowing these exists.
Salons are popular for a good reason - cost is lower, its where people all go and most importantly, most groomers prefer being at a salon, so chances of you finding a groomer is probably at a salon.
But salons poses serious risks that no matter how it's advertised - it is not a real one-on-one service. Salon groomers can work on your dog before moving to another one - that's what they advertise as one-on-one. but there is always going to be other dogs around waiting for their turn or be picked up. This means there is going to be chance of being attacked or dogs being highly alert on presence of other living animals. The solution is caging and isolation, but both of these method introduces new problems. Mobile grooming is the only true one-on-one service that does not risk your dog to other dogs.
Additionally, people have become more and more aware that the cost of taking dogs to the groomers is quite high. The cost of fuel, transportation, time lost does not justify for the savings.
However, this does not stop people from switching to mobile - in fact, salons are still popular for a very good reason. At a salon, people don't see how messy the grooming rooms are - only the shiny front desk and clean looking receptionist. They also won't stay with their dog to realize that their dog might have been stressed the entire time. When people use a mobile service, the reality of grooming exposes - hair everywhere on groomer, messy grooming trailers and witnessing dog stressors during grooming session, which is all present at a salon but simply invisible to customer. The concealment of reality of grooming is what appeals to the public as they do not want to be involved with it.
People that do deeply care about their dog would find mobile grooming comforting as they know exactly what's going on and would find salon grooming lack the transparency.