Their servers can't keep up with the load now. Also, ingress never died off, there's more people playing it than ever and I've seen a ton of new players coming in in the last few days, people that found it through PoGo, no doubt.
They tried as hard as they can to make go kid friendly: no comm chat, no Intel map, no direct conflict like within ingress. The battle dynamic is completely different in go. I'm sure some griefers will think of ways, it seems pretty well thought out.
ISIS has claimed responsibility for this, he may have acted without them but they certainly wouldn't have turned him down had they known of his plans. Perhaps they even did.