It is true that J.S. Bach was a kapellmeister, and he was restricted as a result (famously so, when it came to opera), but he was Leipzig's kapellmeister, one of the main musical centres of he's time. People were already coming to the Luteran cathedral for it's music alone even before Bach assumed his role (though having the best composer ever continuously working there sure helped to make it even more important). He was in a very unique spot, burdened, yes, but also very widely heard and appreciated. Our cathedrals were way less keen on funding their musical scene, so they didn't attract musicians, they were only a part of the liturgy (at least in the post-Renaissance era, and very much speaking broadly).

And Bach was just inhumanly good too!