Because some stuff that gets posted to both might not get as much traction in the larger tribe, where it may reach the top of the other tribe. The up votes are tied specifically to the tribe it was voted up in. Plus with the help of larger tribes it will help smaller more fan driven tribes grow, which in turn produces content for both and encourages growth for both.

And often something viewed only by fans gets a diffrent opinion than viewed by the majority.

I would think eventually /t/gaming would be a starting point for new members to find smaller tribes. Then you would just leave /t/gaming.