I own a boarding kennels, 10 rooms, so not big.
The dogs get our for five 15 minute walks a day plus they get free time in the paddock.
I have 5 dog walkers and 4 home boarders as customers and I have only been open 3 years.
Why don't they use a boarder? Because they don't trust them 
Why wouldn't I use a home boarder?
The boarder can cancel on you if they are ill/family crisis. Kennels are more likely to have staff so the show would go on.
A boarder can only take limited dogs due to the new licensing rules, they get booked up very quickly and far in advance. Kennels are more likely to have room.
Boarders have dogs in their house, they may well have kids in their house. Doors could get opened, there have been a fair few dogs lost by boarders. Kennels are built to be safe - we have a minimum of 2 locked doors between a dog and the outside. Not going to lie, it could happen that a dog escapes but I.would say it is less likely.
As a kennel, I take dogs from home boarders who haven't settled/argued with resident dog/are generally hard work. I have never has to send a dog to a home boarder because they didn't settle here.
As a final point, there are great kennels and great home boarders. There are shit kennels and shit home boarders. Do your homework, look at reviews, meet the boarder/kennel owner, choose the one that gives you the best feeling.