Why Is MLP so mainstream?

Because it has good animation, awesome storylines, intersting characters, adult references, and much more! If you don't agree, watch the second "Royal Wedding" episode
Here's a scence:

My little cousins watch Dora and Sid and I can't stand those shows. They are way different than MLP. Mlp doesn't treat it's audience like a bunch of idiots. Also you really need to watch more than the pilot episode to get a good idea of the show. I'd say at the very least the first five episodes.

If you pay attention to it you'll also find that there are many references thrown in there for older viewers. Although I'm sure your friends have mentioned some of these already.

The main reason I personally (and many other bronies) love the show so much is the characters. They are very 3 dimensional and frankly better written than most characters in shows aimed at adults. In shows like Dora and Sid the characters are very flat, and in my opinion characters can make or break a show. Amazing characters can make a show with a boring plot great while flat characters can make a show with a good plot uninteresting. Fortunately, MLP not only has some of the most amazing characters I've ever seen, but they have intriguing and often funny plots as well.

The reason you need to watch more episodes of MLP before you judge it is the characters. You can't see any of the character development from one episode. In MLP character development continues all the way through the entire series. For instance I started out not liking Rarity but after seeing her develop I eventually grew to love her.

there is also a ton of fan art, music etc. about the show that is truly amazing, and adds to the experience of being a brony.

You also have to understand that not everyone likes the same things. People have different tastes when it comes to what they like to watch. MLP might simply not be your thing.

cuz it's good

If you don't like it, don't complain on the fan spot.

CUZ ITS AWESOME!!!!!! :D MY FRENDZ AGREE!!!!!

Ahem...
This show was created by so many talented and creative people. The first episode doens't exactly give you a good gist of the show. I suggest watching a few more because the show gets better as you go on.

Awesome voice acting, awesome characters, awesome plots (no pun intended). That was there before the huge teen/adult fanbase. Now that they know we're here, they put in references and other things just for us. They have everything from outside references, such as Dr. Whooves and the Big Lebowski ponies, to references specifically at the fanbase, such as Derpy and Fluttertree.

Honestly it is a really great show. It is funny with awesome characters and it actually has a story line. I know its strange how popular its become, but honestly I think its earned its popularity. I first I did not think I would like it, but then I gave it a try and I fell in love with it. Its great and my new obsession as far as television is concerned! Plus I watch it with my sisters. I am a girl too. Also, if you previously watched MLP like I did as a child. This show brings back such good memories! And in my personal opinion may even be better than the original. And as others as said it also has a great cast of voice actors, music, and the production crew is great. The creator of the show helped to work on Powerpuff Girls which is also one of the best American cartoons ever (in my opinion)!