People use them because of their reputation and very dominant brand. Binance's multiple hacks (of BTC and KYC info) work against them in those categories. People also use them because they rightly assume Coinbase Pro has better fiat liquidity, which is a draw for BTCUSD traders.

I think Coinbase analyzed their Pro customer base and realized a big chunk of users was only using it as a fiat onramp. Those users won't go to Binance.US just because Coinbase raised their lowest tier fees 0.35%. So they got taxed, while regular traders didn't really see higher fees under the volume-based tiered system.

Smaller traders (at the lowest volume tiers) obviously should and do trade at Binance, but I can see why lots of traders (who aren't at the lowest tiers) are willing to pay slightly higher fees to trade at Coinbase Pro.