Because it's not a matter of believing this or that. The issue is Walsh's failure to ask the right questions and to follow up on them. Now, I think we'll all agree Walsh is a good journalist. He'd know which questions he should be asking, and which of the answers he's getting don't stand to closer scrutiny. So he's either blinded (by personal likes, a willingness to believe in Sky or whatever) or a sellout.
Not sure about this being due to his believing JV, as Digger suggested. For one, 2009 Wiggins was a vastly inferior rider in absolutely every aspect, so even if he was clean in 2009 that doesn't mean he was clean in 2011-2013. Furthermore, JV has said Wiggins pretty much did his own thing even back in 2009.