Ok, after giving it significant thought this morning, I think I've come up with a reasonable explanation for the question: why do so many boys/men like MLP:FiM?
I think the answer is the following, although I can't really prove it:
Inside all of us, embedded by default in our hearts and minds, is a love of that which is innocent, and compassion for fellow humans. Every person on this planet was brought out of absolute nothingness into the light of physical reality and consciousness, and as such we all have a kind of pity for ourselves and others, as we are subject to the struggles and trials of our ignorant and confusing lives as humans. A kind of fellowship is formed, connecting all of humanity in our quest for the ultimate truth, beauty, justice, and love.
However, throughout the course of human history, we have forgotten in selfish indulgences these feelings of empathy for one another; and have began to see each other not as brothers and sisters in our humanly struggle, but as objects and tools we can manipulate and utilize for our personal gain and immediate pleasure.
But ponies defy all of that.
Presented in a very pitiable and esthetically pleasing forms, these ponies unite us in our feelings of fellowship with other human beings, making present our desire to love and be loved, while showing our uniqueness. Loving and tolerating comes to us because of empathy and compassion for other human beings, but until ponies came along, there weren't many ways we all could get together as we do now. Bronies may not be perfect, and may not realize this themselves, but at least subconsciously something of this matter is going on, and it is so unlike our modern understanding of reality that is confuses us and makes us feel...
different.
What I have just stated explains why anybody can get into the brony phenomenon and why ponies work so well for them. MLP:FiM is great for many other reasons, such as great animation, characters, morals, ect., but that really isn't enough in itself to create what I will soon describe: a fanbase quite like the one it does.
Now, you may say that I'm going way deeper and I'm getting way more philosophical than not only is necessary, but is realistic.
That may be the case, but I'm positive there is truth in what I'm saying. Think about it. This is a show for little girls. It has a fan base. A fan base that has way over 800,000 members (recent survey), including members way outside the target demographic. A fan base with hundreds of awesome musical pieces, stories, and art pieces.
I could be wrong, but something far beyond aesthetics is present here. And that ^is what I think it is. I know it is for me.