Where does the difficulty lie in developing an iOS version of citra?

To my knowledge, iOS is capable of supporting up to OpenGL ES 3.2. So what are the difficulties in developing a Citra iOS version?

You are incorrect. Apple “supported” OpenGL ES up to version 3.0, which is too low for Citra to use. And even then, it was only supported up until iOS 12. Hence, it currently requires the use of Apple’s Metal API for Citra’s graphics. Unless someone was interested in writing a backend only usable by Apple devices, that wasn’t going to happen.

However, we do now have work in progress Vulkan builds. Which, through the use of MoltenVK, can be utilized to use Metal on Apple devices. In fact, there is already someone out there trying to port our current MoltenVK progress for MacOS, to iOS. So, although the Citra team currently does not have plans to create an iOS port, there may still be hope for others to port the project themselves. We’ll have to wait and see.