The issues are margin, trading hours, market data fees, automated trading and options.
Some traders are already on ICE (e.g., Natural Gas and Power traders) and it's more convenient to go ahead and trade Crude there.
Also, and this is big, if you have spreads or cracks on. then you should stay on one or the other exchange so the span gives you margin credit for this. An example is the spread between WTI and Brent. Held either on CME (as WTI - "BZ") or on ICE, as "ICE WTI" - Brent (proper), your margin is roughly two-thirds that for each contract. Held as CME WTI - ICE (IPE) Brent, it's twice that for a single. For the same margin, you can put on three times as much if your spread is between contracts on the same exchange.
CME has a very liquid Globex window available for off-hours. ICE will be much less liquid.
If you stay with one or the other, you don't pay for Market Data on both.
With CME, you can easily robotically trade. With ICE, you must use one of their preferred (and costly) vendors.
CME has a very liquid complete chain of options for each contract.
For 99% of US-based discretionary traders, CME is the best choice.