That's an essay question anon. Luckily I've got time.
It's because in spite of the "lol April fools" of mlpol's creation, the board and its users found something worth keeping, worth holding onto.
Having done so, anons from all over the world threw in to hold onto it for even just one moment longer. Some offered their coding, others offered their time. Others offered their resources and support, and a great many offered their shitposts. But it was more than that. There's nothing universal about the userbase, but overwhelmingly was a perception and understanding that as much fun as 4chan (and other similar places) are, there's things that are untenable. Pol has reddit, bots, and shills. Mlp thinks it's better than anyone, which is why they stick to their generals, to be around 'others' who are better than everyone.
The list goes on. The point is, 4chan over time became subject to influences, groups, and agencies that were adversarial to the fun, and gradually the fun went away. Not a sudden and extensive loss of fun, but a gradual one, a frogs in a pot sort of decline.
The userbase of /mlpol/ knows what can be lost from a comfy and fun place, and being largely oldfags, has determined that this will not happen again. It's not the liveliest place, but everyone is participating, and I don't just mean posting.
Staff consists of able-bodied users who have distinguished themselves as capable, but they're users first and operate with that in kind.
Users police themselves, and adhere to 'the rule' when necessary. Running this place is not a solitary effort,
It's a collaboration amongst faggots who know that much has been lost, but theres much still to be saved. /mlpol/ is not granted, its earned in a manner of speaking. Every day, or every week, or every month, keeping this place how it is and growing it into what it could be is a responsibility.
And anons know that.
A little stream-of-consciousness-esque, but that in a nutshell is why.
And you know what goes great with nutshell summaries? John Elway and the Denver Broncos. People think you just become the Denver Broncos overnight. That's true, but it's more complicated than jus *poof*. It would be more accurate to say that you become Football, and thereby with your newfound American are inaugurated into the Denver Broncos. This is closer to the truth, but is not th e truth, and therefore not Football.
Or John Elway.
The truth is that Football becomes (You) and through your transformation unto Football, you see things clearer. It's almost like John Elway took a squeegee to your balls, which after getting all that nut-cheese off, you find your dick is suitable for ponies. And ponies are very much suitable for a clean and proper penis, which you suddenly have. Just like Adam and Eve after eating the fruit of knowledge, except in this case it is the best fruit, the fruit of Football, you suddenly become aware of everything you have been oblivious to previously and hang you head in shame. This does not last of course, because about .3 milliseconds after experiencing the briefest moment of shame you comprehend that you are now Football. And in this knowledge, your American begins to waken from it's long sealed and dormant state. And as you grow into Football, John Elway looks down from on mile-high and laughs in Football, causing little foil-wrapped chocolate footballs to rain down upon you. After eating a few (but only a few, cuz eating to a stomach ache is NOT Football) you find yourself sitting back and admiring how American you have always been, you just didnt notice.
And that is the moment of ascendance for a Denver Bronco. And that is why John Elway is Football for all time. And also John Elway.
Praise Football, praise American.