In this case, Millar just happened to be in the middle of a statement at a press conference when the news broke. He gave his gut reaction out loud, which is news.

In other cases, it's because he speaks English, and is well-spoken. He used to write a diary-style column in ProCycling, and lots of British, US, and Australian journalists know him. It's not that he's the best person to interview, but he's become one of the easiest (for the english speaking press).

David Moncoutie has always been outspoken about drugs in cycling, but he gets very little English-speaking press because he speaks French. Other than Moncoutie, if you're looking for a clean rider with a clean past to comment on doping, well, you're just going to have to settle for the likes of Millar.