I'm not trying to say hardcore is an easier game mode, it most certainly takes a different approach to be good in. You kill easier, but you die easier. No HUD/cross hairs. Friendly fire. I'm sorry to approach this one-sided, but the thing that gets me here is killing easier. It takes less bullets, your opponent has less time to react, guns that hit harder and faster rather than more accurately succeed. Explosives are crazy insane and shouldn't even be in hardcore to be completely honest. But they still are.
The skill it takes to kill an opponent that can react back and has a chance of killing you even if you are first to shoot is something of a special interest in pro gaming opposed to this same scenario being impossible in hardcore game types. Far less time react, whoever shoots first and hits wins, scorestreaks/explosives/explosive scorestreaks are nearly impossible to counter in hardcore. It doesn't make for a boring game per se, but there isn't much excitement in watching it. Something MLG aims to be: Exciting. Well to me MLG isn't really that exciting except say the rare clutch/comeback. And for most normal COD players that's all they look forward to too. Something MLG knows about it's audience. This is far less likely to happen in hardcore and that's something they have considered.
Whether or not every game mode should be in MLG and all play styles should is a different discussion. I would completely support hardcore and other such play styles in MLG. I wouldn't watch it, but of course the hardcore community would. Different play styles are interesting and exciting to different people.
MLG first and foremost thinks about getting views/clicks for ad-revenue, being exciting to get those clicks, and adhering to the biggest part of the COD community (mainly core players) for money. It's all about the money. It's not really in their interest to truly find the best player/team of them all out of every gamemode ever in COD in every game. It's always the newest COD of the year, the same 3 gamemodes (CTF, Dom, and S&D), and mostly the same teams who are pretty much over hyped and have a following, and lose all the time in public matches and so never use their MLG accounts in public games.
Really as a hardcore player you shouldn't give a damn about MLG. It's a huge cash cow and a terrible community. They idolize people who are rude to their own fans, who play unfairly out of tournament, who use modded controllers to gain an edge, who try to grab as many sponsors at the same time as possible, who hate and shame other styles of playing, who use dislike bots/their fans/their community to hate on other people's channels on youtube and other sites when someone lashes out at them, who only support the part of the COD community that helps them the most, the list goes on and on. They aren't very nice people, and certainly aren't idols, but the hype, the sponsors, their "fans", and the MLG community keep this bloated ridiculous thing afloat. Otherwise it would sink into the abyss and more people wouldn't give less of a damn.