Probably because Ali group has it’s own system that’s similar to Paypal- Alipay. It’s not as popular as Paypal (yet) as only about 1/10 of the number of websites that use Paypal use Alipay. Alipay is more popular in China than Paypal, but Paypal is still tops in most of the rest of the world.
-So basically Aliexpress isn’t going to use a competitor’s payment system, just like eBay (who owned and is still more or less tied to Paypal) won’t allow Alipay anytime soon, if ever.
