Why? US has delayed plans by months before. Who knows what operational reasons they might have had.

Also something of this scale the US probably didn't want to directly tip its hand. So it warned people weeks in advance, specified a false timeline knowing most Americans likely to heed it anyways would probably avoid such gatherings for even longer than that.

Also could have been Ukrainian SBU, the certain parts of the US learned of it or were informed, issued that alert and maybe knowing their chances were worse with that alert out they rescheduled and tried to keep it from the Americans next time.

Edit: Now it's being claimed it's IS(IS) who is claiming responsibility. IS is an arm's length plausibly deniable CIA proxy. They knew what they were going to do, but they may not have known when or had control of when as is the case with proxies (they may have even known when but tipping their hand caused a change in plans and had it pushed back). Needless to say even if one doesn't accept IS as a CIA proxy, the US is ultimately responsible for the creation of the group and its power via their campaign against Assad. Thus the US is responsible once again for the deaths of many Russian civilians.

I would not be surprised if they try again with more, the US would love to turn Russia's attention to Syria, to split it's military operations between that and Ukraine to give Ukraine more time.