She is EXTREMELY popular compared to all the other vocaloids. And I've seen journals explaining why she's so popular, but I can't seem to get myself to make her one of my favorites.
I'd say a couple of my favorites are GUMI and IA. There are more, but I don't really pay too much attention to them even though I probably should. I'm just not a huge fan of Miku. Even my brother is a TOTAL fanboy of her. Well TOTAL is exaggerating, but the only time I've heard him talk about other vocaloids aside from Miku was like 3-5 times.
Just not many people pay attention to the other ones as much. Like for merchandise, if you want something like a wallet for example, it will be guaranteed that you will find something for Miku with many of that kind or many different designs with her on them. Now you could probably find some with Kaito, Megurine Luka, and the Kagamine Twins, but it will be less likely, and when searching you'll find so much of Miku EVERYWHERE.
Not that I don't like her, but she gets so much attention.