Definitions:
Status= Sociological term for a position in one's society, which determines honor, prestige, and expectations concerning the individual's interactions with society. We all have many Statuses. (Examples: Teacher, Student, Banker, Construction Worker, Police Officer, Republican, Democrat, Christian, Atheist, fan of sports team x, member of club y, patriot of country z, etc.)
Master Status= The Status that is most important to a person's identity. (Often a political or religious Status, other strong possibilities include race, gender, sexual orientation, but can be any status)
Minor Status= Secondary Statuses (Any Status that is not the Master Status)
Social Solidarity= The feeling of unity, of "We," among a group of individuals sharing a common Status (Master or otherwise)
Statuses can usually be ranked to a certain degree, from the Master Status at the top in terms of importance, either to the society or to the self, then down through the various Minor Statuses.
It is known that, under adversity, social solidarity among a group of common status holders tends to increase.
Furthermore, it is also known that people tend to rank statuses who's holders have high social solidarity as more important than statuses with low social solidarity.
This leads to the simple equation:
Minor Status + Adversity = Increased Social Solidarity Amongst Minor Status Holders= Minor Status being ranked at Higher Importance
Said adversity need not be constant. A single dose can unite a group nearly indefinitely. While the adversity may fade, the Social Solidarity often remains, being passed down to future generations of status holders as well as to new recruits. This is how nearly all groups in society operate (All political parties, all religions, all clubs, all high school cliques, all sports fans, all nations, all fandoms, literally just about everything).
Applying this to My Little Pony:
"Brony" Status + Adversity (Occurred originally on /b/, still present on other sites but less common now) = Increased Social Solidarity Amongst "Bronies" = "Brony" Status becoming more important.
Add to it the fact that groups with high social solidarity, with little barrier to entry, tend to attract new members (Especially if the group tends to "evangelize" or even if it just attracts attention to itself by its nature [As is the case with the Bronies]) and that new members of said groups tend to pick up on that social solidarity and conform to the status, further increasing solidarity, and you get a rough approximation of why MLP:FIM is so important to so many people.