Zwift is king for a reason. They were one of the early starters trying to make indoor trainers more interesting. The others are copycat companies jumping on once the concept was proven. Today when a potential customer conducts research before selecting an app they almost all go with Zwift. This is because Zwift has the most to offer every type of cyclist. Over 300 miles of roads and growing, thousands of workouts along with the ability to custom design your own. Individual workouts, group workouts, group rides, Pacer bot rides, and free rides. The race leagues have every type category for every type of racer or even non-racer. The upper race leagues have curtailed the questionable type tactics seen in the early virtual race years. They must be good as 20,000 plus racers show up Tuesdays, week after week , to race in the ZRL. Show me an outdoor race that can claim this.
The Zwift app community numbers in the hundreds of thousands. Zwift promotes Cycling in a number of ways. This year women's Tour De France could not have occurred without the major sponsorship from Zwift. Every year Zwift provides two Pro contracts (1 male/ 1 female) for aspiring road racers along with a triathlon team for Kona. If you think you are good enough you can try out for the upcoming Zwift Academy.
Zwift has created a cycling subcategory out of a trainer workout. The other competitors are simply riding on their coattails picking up the recalcitrant riders.