I think there are a couple things going on here. First off, Russia (as in Putin and the government) desperately wants this to be an accident or something they can at least claim to be an accident (and by accident I mean some massive mechanical failure of the plane where it spontaneously broke into pieces during the safest part of flight when cruising). A successful attack by ISIS is the last thing they want, because up to this point Russia appears strong and unassailable in their air campaign in Syria. If this was an attack by ISIS it shows that Russian people have been directly attacked and are vulnerable. So I would imagine Russia would drags its feet as much as possible in admitting it if this is indeed an attack by ISIS, although the direct involvement of so many other countries in the investigation may make that difficult.
The other thing going on is US intelligence. This crash happened in one of the most intensely watched areas of the world. Sinai is the buffer area that lies between Israel and Egypt. In addition to the US's spy satellites, drones, etc, Israel certainly has its own close surveillance of the region as well. It has already been leaked that US satellites detected a "heat flash" while the plane was at altitude. More than likely the US or Israel has direct evidence that a bomb went off in flight, but the information is too sensitive (for example, betraying just how good the US's surveillance technology is) that no one can officially and unequivocally state that it was a bomb.
That pretty much leaves one other semi-realistic scenario, which is that a repair made a long time ago has failed. Again, that is very unlikely, because a structural failure of that kind would happen when the plane is under maximum stress - during the take off and climb. Not when the plane is at altitude and cruising along with very, very little stress on the airframe. Further, pictures of the tail after the crash show the tail is still attached to the fuselage. The type of tail failures in the past with the Airbus (like flight 587) were the rudder, not the entire tail. With the Russian plane, the entire tail section was intact and attached to the rear fuselage, but separate from the plane. That is a gross failure of the entire fuselage itself, not the tail failing.
It's very likely that both the US and Russia already knows for certain what happened - if it was a bomb it would be extremely obvious from examining the wreckage.