Because it would be adding that much more to the total network hash rate instead of convincing that already existing hash to move pools. More hash is more hash, but it would be ideal to not add more to the network to keep the diff lower.
That's it right there...existing miners instead of brand new hash. That makes much more sense for all. I'd expect to see some big movers coming over in the next week.
That's the whole idea, behind my signature. There are tons of AntMiner owners, who are preprogrammed with disaster, and they
don't know any better. They hear Ant, and they figure it's gotta be the best. A little bit of educational promotion, can go along way.
Once they know they can earn more outside of their comfort zone, they'll want to make the switch.