"I wonder why hasn't Roger Ver and the company - ViaBTC, Antpool, Bitmain, rolled out BCH trading pairs earlier?"

gee, me too.

Bitcoin Cash (BCH) is Turning 4 (Months) – A Look Back- https://www.allcryptocurrencies.news/bitcoin-cash/bitcoin-cash-bch-is-turning-4-months-a-look-back-2/

Thanks to BTC core, BCH has done in 4 months what took legacy years to accomplish. (futures trading(mass adoption), ONCHAIN SCALING)