Why has BCH not received a 51% attack yet? BCH right now has around 10% of BTC total hashrate.

There are multiple pools that control 10%+ of the total BTC hashrate why haven't any of those just allocated a few hours of hashrate to do a 51% attack similar to the one we saw recent on BTG. If they have 10% of BTC hashrate then it would only cost them around $60,000 per hour to control the network (to pay the users using the pool)
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.