I'm of Tea Supremecist's opinion...it's mainly personal taste, although a lot of other factors come into it. Corportate manufacturing has always been at play. For lyrical content, the pop labels specifically cater towards the most universal tastes (hedonism, sex, happy-go-luck clubbing, chicks, and riches) in a lot of cases, filled with the catchiest hooks by the trendiest producers they can find.
But I do not stand by the statement that all mainstream music sucks. Even manufactured music can have its merits if the producer did a good job, if the singer poured his/her heart into, etc. And yes, the occasional mainstream artist can actually have some degree of substance to their work; Prince, Madonna, even Eminem, whom the OP mentioned here, all have had some gems in their extensive musical catalogues...their commercial success doesn't make their music any better or worse than what it is. Similarly, you can find really bad, alternative and less mainstream music. Trust me, there's a LOT of that.
Ultimately, yes, it all comes down to personal tastes, and mainstream music appeases the majority of people's tastes, hence it being mainstream.
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