Please educate me why developing CVN is significantly more difficult than EM Cats?
Because the sheer output that a CVNs nuclear reactor must be capable of, as well as its reliability, and its role in a CVN (providing propulsion for movement, providing electricity for all of the ship's subsystems) is far more demanding and central to virtually every aspect of a ship's function than EM catapults are.
China has has SSN for a very longtime, since 1974, almost 50 yrs ago.
So what?
Look at the output and characteristics of something like A1B or A4W and compare it with those of submarine reactors from 50 years ago.
Not to mention China's experience with submarine nuclear propulsion hasn't exactly been smooth or cutting edge either. Sure, China would have made advancements.
But that's like saying just because China put its first satellite in orbit in 1970, that it can produce super heavy rockets without considering it to be a challenge.
I thought Nuclear reactor on subs is significantly more difficult than on CVN
Not necessarily.
Besides, it's not like all nuclear reactors for submarines are made equal.
Just because they have experience with nuclear submarine powerplants doesn't mean it's "easy" or "smooth" to develop a nuclear reactor for a carrier. As I wrote before, the scale of output, the need to test its reliability, is vastly different.
What it means is that we can dare to entertain the idea that they might be able to pull it off in the first place.
and also currently China is one of the world leaders in Nuclear power plant technology
China's nuclear industry is okay, sure.
But that doesn't mean developing a nuclear reactor for a 100k ton carrier is easy.
Even for the US, developing a new reactor for the Ford class was a significant undertaking.
China is essentially trying to approach a powerplant similar to Ford (or at least Nimitz) class in one go, without ever having developed a naval shipborne nuclear reactor of similar scale before, with minimal room for error.
It's doable, but it is also a massive challenge.
To answer your question in a simple way:
China's nuclear experience doesn't mean developing a nuclear reactor for a carrier isn't still a massive challenge.
The centrality of a nuclear reactor to every aspect of a CVNs function (from the mundane to the mission essential) and the sheer output needed from such a reactor, and the demands for its reliability, makes it more of a technological challenge than EM catapults were for China at the respective times.
You are a PLA watcher.
Don't underestimate the challenges of developing capable and reliable powerplants, no matter the domain or platform.
