My guess would be that the three-day timer is the first version to test the waters. Put it out there to see if there are any unexpected problems. And if everything's peachy, lower it in some future release to make it more secure.
If they went with something more aggressive (like 24 hours) it might annoy more users, especially those who keep their phones locked for a while but don't use them constantly
I imagine the number of people who don't unlock their iPhones in ≥24 hours is a vanishingly small percentage of the userbase. Especially since with new iPhones, just looking at the lockscreen with your face unobscured is enough to unlock it.
Users that have an iPhone just for work would be affected with a 24h check. Even with 72h they might be affected, but it would be fewer (if you assume they work Fridays and Mondays)