Been trying to figure out the exact reasons why I went from multiple chars into very few. I think I'd put it into two main reasons that are part of the "how?":
Not being happy about having newbie characters. I had a bunch of characters, but they all were trash in quality compared to people that had their characters in full raid gear and the like. There was nothing special about my characters. Dime in a dozen.
Once part of an actual working guild that does things together, you end up playing character(s) that are suited for the group content. If your guild is looking to do heroic raid, you can't just bring in a brand new character. You're somewhat tied to the requirements of the group/raid, and it becomes more about playing into that rather than your own whims.
The first reason is kinda just vanity. Wanting to be bit special. Not the most noble of reasons, but hey, it's still there for the most of us.
The second point is probably the larger reason. I joined a guild, got into raiding and got some gear through that. The people in the guild were good company, so it's fun and exciting. How could I suddenly switch to something that's not capable of playing that content? The content gets challenging. I might still roll up an alt, but when it's raid time, it's back to the useful character. It's no longer about really enjoying the class alone, it's about the progress in raiding and dungeons. It's about being part of a team that struggles together to beat the next big bad. The low level alt has nothing to do with it anymore.
Gearing up a character takes a huge amount of time. I'd like to say that if I were to start a level 1 character now, it would take 3 months before it's at the level where I'm used to having characters these days. It's just not worth the effort in my eyes anymore.