Because of the cost, specifically the opportunity cost.
If a miner has 10% of the BTC hashrate, then if they pointed all of that mining power to BCH, they would still be losing money. With 50% of the hashrate on BCH, they would mine roughly 50% of the blocks in a day, which is 72 blocks. At 12.5 BCH per block,
72 * 12.5 = 900 BCH. Convert that to BTC at the current exchange rate (0.135 BTC/BCH)
900 * 0.135 = 121.5 BTC.
However if the miner keeps mining BTC, at 10% of the network hash rate, they would mine ~14 blocks per day. With at least 12.5 BTC per block, a miner would be making
14 * 12.5 = 175 BTC per day, much more than if they were mining BCH.
Thus a miner will earn more money from mining BTC rather than mining BCH. Performing a 51% attack on BCH isn't really worth it; all you can get is possibly some double spends, and less value in BTC.