Because Selena's band was looking for a guitarist for their band and Chris needed a job in a band

Mostly, alternative, but other people would disagree.

Led Zeppelin is the greatest freaking band in the history of the universe.

I disagree as well; they are certainly good but they've never grabbed me. I prefer Deep Purple, who I think are awesome.

There is no way to answer this question objectively, because of course the band's fans will say they are amazing and the best band ever, while fans of other bands will disagree and say Big Time Rush are over-rated.

Many people would disagree, it is a matter of personal preference.