Well, I'd say it's because people that travel in groups can get a better deal by booking an Airbnb instead of booking multiple rooms in a hotel, and it also gives them the benefit of having shared space (living room) and a kitchen.
It also usually makes sense for longer stays, say a 30-day stay where the Airbnb's usually grant a ~20-30% discount, which makes them cheaper than a hotel.
Other than that, I agree there's little reason to choose an Airbnb over a hotel.