Goh is undeniably the biggest Mary Sue in the series.
Honestly because Goh is Creator's pet and Goh catching Suicune was controversial enough.
The problem with a character like Goh is that he is literally becoming a monster of the creator's own design. Nobody can stop him. It's to the point where they can throw anything they can think of at him and he'll just steamroll over it. Basically, we're in a situation where the writers have effectively lost control of the story. Because if they're even going to PRETEND there's any realism to anything they do from here on out to challenge Goh, it won't work, because he'll stop it.
The major concern here is that I've seen this happen in D&D campaigns, and it never ends well, for either the players or the DM. In D&D when PCs become too powerful, the DM almost universally has no choice but to wipe out the party. And it's usually unfair when it happens, like using the Apocalypse Stone adventure, which is basically the Kobayashi Maru of D&D (ie. the no-win scenario). It's an adventure where the PCs are guaranteed to die at the end, because the book basically says, 'Well too bad. You beat the big bad, but the world ends and everyone dies anyway.'.
My concern is that the creator and writers are going to have to come up with some way to remove Goh from the anime, and do it in a way where the other characters will never mention him again. They probably won't kill him off, but people are going to raise the BS flag, no matter how they do it.
What I think is going to happen - and people won't like it - is Goh is going to finish this 'Mew Mission' that he's on (all the dumb little tasks they want him to do for whatever reason), and the people who arranged that are going to leave with him to go find Mew, and that's where we'll never see him again. It'll be Kurt all over again, for those of you that remember the GS Ball.
To answer the topic question, the bottom line is the reason that Goh can catch Legendaries and not Ash, is because Goh is still a temporary character. Regardless of what websites and even the show call him, at some point he is going to leave. Ash on the other hand, won't be leaving. So giving him a Legendary would cause long term problems with his character, whereas with Goh, any issues that we're seeing, catastrophic though they are, are still only temporary and will largely end when he leaves.
Now if they wanted to have some real guts, they could do it another way, and it might even salvage Goh's character. Have the 'Mew Mission' be part of a dupe. Have the people who arranged all this turn out to be villains, and have Mew be a lynchpin for the balance of the world, and catching it somehow throws everything out of whack. Then we see apocalyptic consequences of Goh's actions as things are falling apart everywhere, creating another sort of 'Darkest Day' and forcing the protagonists to go on a journey to fix it by eventually returning Mew to where it belongs. Then afterward, maybe Goh has a moment of self-reflection. Maybe he realizes that catching every Pokémon isn't the right way to go about things. Maybe learning to understand and respect them is more important. Perhaps, his whole countenance changes as a result and he dedicates himself to learning more about the world, instead of just wanting to throw balls at everything that moves.