I think many miners have more faith in mining bitcoin over the long term. Also, I use Genesis Mining, and they don't offer the choice of mining BCH (not that I would). I think there's probably a lot of mining operations like that. They won't be offering the option of mining BCH. This kind of adds a stability to BTC that BCH just doesn't have. I also think that the moment BCH becomes less valuable to mine, all those miners will switch back to BTC. We'll see I guess. Thanks for your comment!