A difference in priorities
At the end of the day, the two are more similar than different but the major divergence between Bitcoin (BTC) and Bitcoin Cash (BCH) comes down to ranking of priorities.
In its current construct, design, and roadmap, the BTC community is focused first on keeping Bitcoin decentralized so that it can remain as a public, permissionless, highly censor-resistant network that anyone in the world can use and build on top of. Fast, cheap payments are a secondary priority.