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.
Your phone knows how often you normally use it. It could do something intelligent based on a usage pattern to restart when there’s an unusually long gap.
I can see this being too hard to explain compared to “reboots after 72 hours.”