Because gyms here rarely house you, a long stay means renting properly, and the rental market is deep at every price point. Where you live decides your commute, and your commute decides whether you actually make the morning session.
It also suits people who are working while they train. The infrastructure for that is better here than anywhere else in the country, and the timetable of a city gym tends to accommodate it.
Stadium access is the underrated part. Being able to watch good fights regularly changes how quickly you understand what you are being taught.