It's a number of things...
- incredible stadium
- Caxton St and surrounds for the drinkers
- Plenty of activities outside the ground for the kids and families
- pre game laser and fireworks show for night games
- half time show with pretty reasonable acts
- big crowds pretty much always (brings more atmosphere which brings more people which brings more atmosphere etc)
None of this is exclusive to the Broncos but they just have the whole package and you see so many people at Suncorp that you wouldn't expect to find at a Rugby League game. They've made it a "cool" thing to do.
My girlfriend is Japanese and knows nothing about RL but she gets hyped up when I bring her along a few times a year. She's really entertained outside of even caring about what happens on the field.
I'm also a Phins member and they do a good job, but they don't have it nailed like the Broncos just yet.