They clearly thought about other rules and modified those.

My guess is that they wanted to creat end-of-game/end-of-half, opportunities in close games.

Most of the time, a fair catch takes place outside of FG range. Even from the 50, that’s a 60 yard kick. Penalty yards can bring it into play but you typically only see a fair catch take place on the kicking team’s half of the field when they’re backed up near their own goal line. The rule provides a scoring opportunity that incents the kicking team’s to take more risks to get away from their own goal line.

I wish the rule were still around in college as it’s not that uncommon for teams with a lead to end up backed up near their own goal line to run into the line 3 times and punt it away. Even if you’re down more than 7, if you can fair catch the punt in range, executing a free kick for the FG if you’re down 10 and then attempting the onside kick would be preferable to taking time off the clock to try and get the FG.

For example, Denver down 10, attempted a 55 yard FG on 1st down in an attempt to preserve time and then try the onside kick knowing they’d need a TD at that point. If that had been after a punt and fair catch even if from further back, that would have been brilliant strategy to execute the free kick and then go onside kick.

I’ve got to imagine your odds of making a long kick go up dramatically with no rush and no need to worry about getting the kick above the block. In the NFL there are kickers you might consider on a free kick from 70 yards under those conditions.