It is a fashion.
I cut the OTI blank to 4'8" to see how it works compared to Hot Wei World rod, not because I believe the length is right.
Every body has different physics and preference to fight tuna and the preferred length of rod shouldn't be the same.
Years ago, stand-up tuna rods were getting shorter and shorter believing shorter rods give more leverage for fishermen and they used as short as 4' rods. Nowadays, the length of standup jigging rods are not short as it used to be.