There has been some speculation about why. Here are some of my thoughts about why I think this might be happening.
1). Sound quality (internal hardware)
Beats has managed to mass produce high sound quality with low cost of goods (Cost of Goods) keeping a high margin and delivering a mid-priced product. Since Apple isn't known for good audio quality it's possible they purchased Beats to acquire the Beats hardware team.
2). Beats software Algorithm
Beats has a algorithm that's allows users to input explicit subjective data on how a song makes them feel and find songs based on subjective data. Spirit has been making moves this way but they are relying on curated lists to do this. Beats is boldly asking users this data and, in fact leveraging it as a core feature of the user interface.
3). Beats is a marketing machine.
They have major celebrity, tastemaker and music influencers pitching their product. Importantly, they have captured the attention of the "lux" seeking urban market which has enough disposal income and social capital to have taken the brand from 0 to a consumer maker leader in such a short amount of time.