There's nothing blocking them other than their own internal policy and hubris. They want the control of everything being re-skinned Safari on iOS
The reality is, even though it's now theoretically possible for Chrome and Firefox to be properly ported here, it's practically impossible. At any rate it's not happening yet. Last I heard they decided against it as it would require all of the coding to be done within the EU! So Apple is still effectively blocking alternatives on its platform even here.
It would be nice if the USA and Japan also pursue their own anti monopoly charges and force Apple to open up across the world