What's magical about the number 90? It was created by man and represents, although vaguely at times, how large a pronghorns horns are, but why not 85? Why not 95? Currently, we can say that 90 is significant because it represents roughly the top 33 of 1900 antelope entered into B&C's record book (12th edition). That's the top 1.7%.
Another way to figure it - if close to 3 million pronghorn have been taken by hunters since guns were invented then 90-inches means the top .001% for all-time!
For a method we can more easily grasp let's compare pronghorn with elk and mule deer.
The top 1.7% for antelope is 90-inches or more
The top 1.7% for typical elk is 419-inches or more
The top 1.7% for typical mule deer is 212 inches or more
The top 1.7% for non-typical elk is 450-inches or more
The top 1.7% for non-typical mule deer is 306-inches or more
Remember that these numbers are only relative given that the B&C Club has established their record book minimums on a relative basis across all species. Also note that these numbers are derived directly from B&C's record book, thus eliminating a lot of potentials trophies that were, for one reason or another, not entered.
I guess a more accurate measure would be to take the top 1 or 2 or even 5% of antelope, deer, and elk that have ever been taken.
One other thing to consider: I've mentioned this briefly before...an antelopes score is usually reflective of the horn's actual weight (which I believe should be used to determine absolute size), but sometimes there are bucks that score a lot more than others while the others weigh a lot more. This means that, though the inch measurements do give a good approximation of the bucks size, they are by no means the end all of measuring a bucks horns "true" size.
What does all this mean? It means that, in this world, the more rare something is the more valuable it is. In this case, the larger an animals horns are, the more rare it is and thus the more value it holds.
So...why 90? Because it's rare and rare things are beautiful to behold.