On October 12, 2019, Eliud Kipchoge became the first person to run a marathon in under two hours. Why was that performance not the world record, and how was this run different from the Breaking2 project?
Because the IAAF applies a few restrictions for official marathon records. Among these is the absence of pacemakers. Kipchoge had dozens of pacemakers, none of them going the full distance, but being substituted after a few kilometers.
This is similar to why Kipchoge did not earn the World Record at the 2017 “Breaking2” event: the Ineos 1:59 Challenge consciously bypassed a number of IAAF rules about marathon competition in pursuit of the sub-2:00 clocking. The IAAF, which ratifies marathon records, only accepts marks from compliant competitions, and would disallow this performance (which some commentators have called a "marathon exhibition") if it was even submitted.
- Open competition: the winner of the Vienna “race” was ordained from the start. Unlike Breaking2, where several athletes started, Kipchoge was the only athlete to cover the full distance in Vienna. The IAAF would require other competitors under the same rules. (It's dubious if Breaking2 would have been acceptable in this category, but the Ineos 1:59 Challenge didn't even try.)
- Pace assistance: Kipchoge benefited from several dozen world-class pacemakers. While pacemakers are not inherently against the rules, the Vienna setup with pacers joining the race in progress is. To be allowed for record purposes, pacemakers need to start the race with everyone else. Kipchoge also had the visual assistance of a pace car projecting a guide laser on the course; for that kind of assistance to be allowed for record purposes, it would need to be provided for all competitors. (This is also an issue with the pacemakers, who held a formation designed to benefit Kipchoge which would not have been available to other competitors.)
- Equipment. Whether Kipchoge’s shoes provided unacceptable assistance has been a topic of debate, but even if they did not, they were an unreleased prototype, and IAAF rules require that comparable equipment should be available to all competitors.
- Additional support: Kipchoge had more-or-less unlimited access to liquids (a carbohydrate beverage) supplied by a cyclist throughout the run. For record purposes, again, all competitors would need to have this kind of access, and that's difficult enough that it's not done.
Considering that Kipchoge currently owns the ratified World Record (from Berlin 2018), I doubt the non-ratification of this performance will bother him much.