MK was recognized as field clearing for Snake long before people realized he was so dominant -- that really began when they figured out how to fight Snake with him.Ummmm, I'm pretty sure that isn't the case. From what I remember, MK was always ridiculous, except that snake countered him. M2K played DDD, then MK figured out how to get around snake. At around the same point M2K switched over to MK, probably (the way I see it) because he realized that MK is the only way to go if you want to make money. It's not like m2k was the magic "make meta broken" pill.
As far as M2K and MK, he doesn't want his source of easy wins taken away and recognizes no other character is nearly as effective. I don't blame him for this, except that it's selfish and these discussions are about what's best for the tournament scene -- having him claiming all these people who can beat him when he's MK (But haven't really done so when it counted, for whatever reason) is just misleading.
As far as why should he switch from his main, he's the one claiming other people can take down MK. So shouldn't he be willing to utilize those counters to thrash a few top level MK's in tournaments to prove his point? (Yes, it won't be as "fun" as MK but it would demonstrate that other characters have even-close to even matchups) It should be perfectly safe, no worse than him going MK vs. MK and he'd have the advantage of his opponent not being as familiar with that character as they are with an MK ditto. But he sticks to only using MK. It casts a lot of doubt, in my mind, to most of his claims that MK isn't as dominating as he appears to be -- and I suspect M2K knows he would lose more frequently using anyone else, and really just doesn't want that to happen. (Note: I am not saying he will lose anything close to a majority of his matches. He would still remain the best, but even the best can lose rounds occasionally and I think that rate would slightly increase without MK as a crutch for him)
And why is MK fun? Because he's got a broken number of options. For instance, I was reading in a different thread that he can end his shuttle loop at its top. That means when you shuttle loop, if something goes wrong, you just break out of it at the top rather than completing and facing whatever punishment you should have earned yourself for misusing it. Or you can attack right at the end. Or you can glide a bit and then attack (Or just land). His up-B recovery move alone exemplifies the problem -- he has a stupid amount of flexibility compared to everyone else. It would be awesome if every character had this many options, and would make for a much better game...but they just don't, and so MK is a problem.