The recent Bitcoin halving that affected many mining pool also affected BTC. As a result of this halving BTC also halved miners reward from 12.5 BTC to 6.25. this will in no doubt affect the miners and also the Bitcoin hashrate on BTC mining pool.
Before now BTC used to be the largest Bitcoin Mining pool for years. Since it came live, it has been maintaining a steady growth to the extent that it was ranked number one and has accounted for more than 18% in terms of bitcoin hashrate.
However BTC has been overtaken by other pool whether it will be able to regain it's first position in bitcoin hashrate only time will tell. But I know that considering that fact that BTC has the advantage of mining with bitmain machine which is known for fast transaction and lots more, it's likely for it to find its way upback.