A couple of reasons:

1. Bitcoin has the branding which means anytime somebody wants to buy Bitcoin, which is the cryptocurrency everyone has heard of, they will contribute to increasing the value of Bitcoin over Bitcoin Cash

2. Ethereum vs. Ethereum Classic has illustrated to us that the minority fork has limited probability of taking over

3. Lightning adoption without SegWit is possible, but less ideal which means Bitcoin Cash will likely stay with on-scale solutions which can be a slippery slope. If you screw up Layer 1, it's difficult to fix - even if Lightning sucks, at the very least you can ignore it by just operating on Layer 1. The point being, once Bitcoin implements off-chain solutions, Bitcoin Cash will likely become less relevant as fees go down.

4. Bitcoin's primary source of pricing by the market, as I have discussed before, doesn't come from it being a transaction medium - which means that Bitcoin Cash's superiority in this regard (in terms of fees) really shouldn't matter in terms of market cap.

5. All reference prices for Bitcoin futures currently use 'Bitcoin' which will be difficult to change

6. Roger Ver is NOT a good face for the cryptocurrency. He has the passion, but he is unable to control his emotions, often can sound way out there, and struggles to be mature in his representation of Bitcoin Cash. To be fair, many faces of Bitcoin struggle to be mature as well, but it is not Bitcoin's burden to convince precious dollars to come over. Ver should be holding himself to a higher standard than the rest which he is struggling to do unfortunately (or fortunately depending on how you look at it).