Why not BCH?

[ShadowCrypto]

I think a lot of people argue Bitcoin is better than Bitcoin Cash because there is a lot more transactions happening on chain.

I believe a majority of Bitcoin on-chain transactions are just speculators who are sending Bitcoins between exchanges to provide liquidity, make money from arbitraging, or to purchase alt coins.

Why are they using Bitcoin?

Because BTC dominates trading pairs.
I wonder how things would change if exchanges start using another coin or token that is cheaper to transact on chain as the main trading pair. I think you would see BTC lose a lot of value.
Because then every trader, arbitrager, etc. will use the lower fee coin to move on chain.
$5 fee on Bitcoin right now