Because Zayn tries to have demeanor of an R&B/Hip Hop star meanwhile his image has been contrived so he can be the "bad boy" to 12 year old girls (and a few confused gays here on ATRL who don't know there are better members in One Direction).
Zayn told the truth, let's not forget that when the Wanted first started only Max & Tom knew how to sing properly live...
Ok sure.
And of course they were the only ones, they had the most experience. Max went on X Factor along with Tom and both had side groups. Nathan was still in theater, Jay was still taking up dancing, and Siva was modelling.