My guess? The FCC is probably close to deciding and Obama knows which way they're leaning. So he wants to build some distance between the DNC and the FCC, before the FCC inevitably announces they're going to let the ISPs do what they will.
If Obama says nothing, Net Neutrality will continue to be a sub-headline issue even if/after the FCC allows it to die. Inasmuch as anyone notices, it will reflect poorly on the DNC in 2016.
If he says something now, the reactionary portion of the GOP will take the opposite position as a knee-jerk response. It will become a larger news item and the GOP will have inherited the fallout. Inasmuch as anyone notices, it will reflect poorly on the GOP in 2016.
That he didn't say anything prior is probably either because the FCC only recently notified him of their position, or he wanted to avoid any blowback on those democrats who were already running tough mid-term re-election races in conservative districts.