The fundamental reason for Kipchoge's time not "counting" as a record is the nature of the Breaking2 project. Speculation about the possibility of a sub-2-hour marathon in the past focused on the various factors outside human potential which limited previous times. The Breaking2 project aimed to minimize these factors and reach for a best possible performance, and in the process knowingly discarded several conditions required for a record:
- Perhaps most significantly, the runners were escorted by a rotating crew of pacemakers, in shifts of six, who would run goal pace in a phalanx in front of Kipchoge et al to protect them from wind, domestique fluids and fuel, etc. While pacemakers are often present in record-setting races, they are required to start with other competitors, and once they leave the race they can't rejoin. Breaking2 pacers would be replaced by fresh runners mid-race, and each "shift" got 30 minutes of rest before returning to the run.
- Record "races" generally require a certain number of finishers; Breaking2 had only three.
- The lead car in Breaking2 offered some assistance to the athletes, projecting a line for goal pace on the course and employing a large video screen to both feed data to the athletes and (to some degree) shelter the athletes from the wind.
Other factors are not specifically against record certification standards but were different in the Breaking2 effort. These changes get at the very definition of what counts as a "marathon":
- The pacemakers employed a triangle formation to shelter the primary athletes from any headwind; this is legal in normal races but not usually orchestrated to the same degree.
- The course was a 2.4km loop on an auto racing track. It was certified to IAAF standards, but normally big-city marathon courses can't be fine-tuned to the same degree to avoid hills or sharp turns.
- The athletes involved were all subject to normal doping controls, but the effort's stated "how fast can we go if we don't limit ourselves" philosophy raised a lot of eyebrows among doping cynics.
In short: Kipchoge's remarkable clocking definitely redefined what's known to be possible. But aside from covering the same distance, it didn't have a lot in common with what we consider to be a "marathon", and some of those factors made it impossible to certify the performance as a record.