An interesting move. Something that drives me crazy when interacting with China, is that I can only use American payment methods to make a payment from Europe to China. I pay a fee and the merchant pays a fee both to the US. It just sounds broken. I hope this is a step in eventually resolving this.
I would imagine a big reason for not making the effort was convenience. If they make Unionpay more convenient to use then people will likely be more likely to switch.
Of course, I could get a Chinese Unionpay card/account. Now each time I want to make a payment, I need to move money there, and can I even do that easily from a European bank account?