He probably doesn't think it's that good.
This. Which it ain't. I've never really understood all the love that song gets.
From the perspective of someone who's in a band, although admittedly with a mere fraction of the songwriting experience over time that Green Day has had, I can honestly say that there are songs from when we first started that I would be somewhat embarrassed to play now. We thought they were amazing at the time, but over the months, our songwriting has gotten better, and we've come to realize our first efforts aren't that great compared to what we came up with later. I'd imagine this is how Mike feels about Christie Road, especially with new material that he wants people to be excited about.
Also, somewhat related. We touched on this a little bit in the thread discussing the tour setlist so far, but the majority of the people at shows are the people who know those big hits, and little else. Again, I can say from personal experience that playing a song of yours and having people sing along who know the words is an amazing feeling. Of course the band needs to play a setlist that appeals to the lowest common denominator, but they also probably want to play a setlist that gets everyone singing along and jumping up and down. Yes, the hardcore fans love Christie Road, but Green Day can't design the vast majority of their setlists for the hardcore fans. They have to cater to the masses, and dare I say, they might be happy to do that if it gets them a riled-up crowd.