I can think of two reasons: preserving the integrity/value of the cabin and keeping seats open for non-rev employee travel.

If CO were to regularly upgrade passengers with miles into the cabin, even within the 72 hour window, consumers may not buy the seats ...and just depend on the upgrades. By doing so, the BF cabin is filled mainly with people who paid for the privledge ...and can be serviced as such.

The 72 hour window also provides a clear view of available inventory for non-rev traffic. Free/reduced travel is a perk of airline employees they deserve...so I have no gripes about that.

I just wish the Platinum pool were a little more elite ...and wish that CO would recognize the elite of the elites a little better with more flexibility --like the waiver of the 72 hour rule.