Is it because it's what came on your car from the factory? Because it's what you always used? It's what your friends recommend? It was on sale? The salesman recommended it?
My favorite tires have been off-brands. Twice, I have bought Futura 2000 tires from Pep Boys for small commuter cars and both times the tires have performed much better than the factory tires. Surprisingly, they lasted considerably longer as well.
But wait! Pep Boys doesn't make tires.
Futura tires are actually made by Cooper tires.
Why not just sell them as Coopers? If you know the answer to that, please let me know.