Every Motorsport title has gravitated towards 8 minute races. Every Horizon title has gravitated towards 3.
I’m sure they have internal data that says that this is the optimal length/cadence for player retention.
One of the key factors isn’t necessarily the races themselves, but rather how many races/sessions one can reasonably expect to have in an hour of play (including loading, matchmaking etc.). At the moment a person doing Short races can get 4 in. In Horizon they can get in 10-12.
If you only have long races, and you get wiped out on Lap 1 (or your game crashes), you could have a long wait for the next race for example. Likewise if you power on your Xbox a few minutes beyond the scheduled race time and miss a slot.
For Short races I think the length is just right, but the entry closing time between races is 2 minutes too tight. I reported that here: Featured Multiplayer - Entries for Short races close 2 minutes too early