Here's a little explanation for those who DO fail to "understand why".
There are pretty much two reasons that MLG was left out of Halo 4. You see, MLG, 343i, and Virgin are all businesses. In this case, 343i and MLG have had some history with each other (at least the Bungie employees who are now with 343i have) and MLG has become nearly synonymous with Halo. Having MLG take up the role for Halo 4 that it did for previous Halo releases would be the "safe" bet. And businesses hate to gamble. So, as I said, there are only two reasons that this would have happened.
1. Negotiations break-down. This could come from either side, or both sides. Either MLG's demands were too stiff and they wouldn't bargain, or 343i's demands were too stiff and they wouldn't bargain. Then Virgin comes in and bargains with 343i, met their demands, and bob's your uncle MLG is left out in the cold for Halo 4. This seems the more likely after the MLG/Reach debacle.
Or...
2. Virgin made an offer that MLG couldn't/wouldn't match. In a nice little fairy tale world, 343i would have still gone with MLG despite this. However, it would be a horrible way to conduct business. If you had an apple and a guy who you knew offered you a dollar for a slice of the apple, and then another guy who you were familiar with but didn't know came and offered you one hundred dollars for a slice of the apple, which should you go for? If you answered the former, I wouldn't suggest you open up a business.
If Virgin is able to give 343i more of a profit than MLG during Halo 4's lifetime, then don't expect MLG to come back in Halo 5. Bungie didn't take much of a chance with MLG back in the day, because MLG didn't really have any competition. Now, however, if MLG can't compete with Virgin, then the days of MLG are coming to a close.