Alex, Glaze, etc. have the followers they do firstly because they're incredibly talented, but also because they were around at the beginning, where there weren't as many options for people to listen to. Not to mention it's been, what, a year and a half/two years since this site and most of the popular musicians started their careers in the fandom.
You must also look at the talent curve as a result of what effect time in the fandom has. Currently a lot of newer musicians are actually producing the same level of quality (sometimes higher) songs that most of the top musicians were putting out way earlier in the fandom. The big difference, is while you're improving, so our the top musicians.
What this does is create an exponential curve. The top musicians started off at a decently high level of talent, and have improved at a steady rate. The ones shortly thereafter started off at a slightly less level of talent, but have improved at a faster rate due to learning from the top musicians.
The large bulk of new musicians with the announcement of Balloon Party started off at a lower level of talent, so they need to improve at a faster rate to gain visibility.
Lastly, the newest musicians started off at the same level of talent as the above group, yet they have to improve at a much, much higher rate in order to gain visibility.
Best illustrated by a graph:
*Note, this graph is showing the concept, and is not meant to be an accurate representation.*

Based off the graph, it is expected that a few years down the line, every group that encompasses musicians from 2011-2012 will meet at a point, but that's only under the assumption that these groups can consistently improve at a faster than normal rate, which is unlikely to happen, save for a select few.
Long story short, the longer you've been in the brony music scene, the more likely you are to be known, and for the musicians starting today, it's very likely they will always be overshadowed by the other groups before them, unless they can improve at a higher rate.
Groups were based off the Top Brony Musicians list from the Brony Musician Directory: http://bronymusiciandirectory.blogspot. ... cians.html