Now, Bitcoin Cash is second in term of market cap. What is happening? Here are maybe the reasons behind the decline of bitcoin price.
Bitcoin Rejects 2MB Blocks (Segwit2x)
Current bitcoin block sizes are limited to 1MB in size. This limit cannot be changed without a hard fork. And to prevent Bitcoin from temporarily or permanently splitting into separate payment networks ("altcoins"), hard forks require adoption by nearly all economically active full nodes (Source). Why need to modified from 1MB to 2MB? With 1MB block size, transaction fee is high and the transaction confirmation is slow. To solve this problem, bitcoin need to upgrade from 1MB to 2MB as proposed in SegWit2x. However, SegWit2x hard fork has been called off. With this decision, bitcoin may become a digital gold which it is hard to use for daily payment.
South Korea
Bitcoin Cash market is heavily concentrated in South Korea, with three major South Korean trading platforms accounting for around 53 percent of Bitcoin Cash trading. The rest of the trading volumes are composed of Bitcoin Cash-to-USD trades, not Bitcoin-to-Bitcoin Cash trades. (Source) South Korea may help push bitcoin cash price to increase.
1MB limitation is the real problem. When bitcoin becomes more popular, transactions will surely increase. As result, bitcoin will be the victim of its own success.