BCH has not much promotion, not much value, not much support in the community. They don't have as many exchanges, not many wallets to choose from, no merchants that accept it. When people think of BCH they see Bitcoin's little brother that's just waiting for the older one to die so it can take the crown.
I think the idea here for Bitmain was to test the waters and experiment with a Bitcoin hard fork. The old way with XT and Unlimited clearly wasn't going to work. So they tried an opt-in hard fork. This way, instead of trying to "force" the community to upgrade, they can try to show a gradual, organic adoption and/or overtaking of the legacy chain.
Right out of the gate, BCH has a lot of exchange support. Bitfinex and Poloniex are huge, and Coinbase will give their customers their BCH. Maybe they'll open markets for it in the future. Bitmain can also gradually point more and more hash power at it, making it look like miners are responding to market demand. As long as its profitable per the exchange rates, it will look plausible to the market, and the public might further buy into the narrative.
