It's nothing more than playing catch up.
I'm company X and I managed to build an aircraft for a certain mission at the lowest ever MTOW.
Company Y will try to get down to that MTOW to be able to compete.
Check the MTOW of the Airbus A320-100 and the Boeing 737-400, to save you time, they are 200 pounds apart.
But the A320-100 didn't sell well, the A320-200 did.
Guess what, the 737-800 matches that weight.
Now there's something called Weight Factor.
It's SQRT(MTOW/50), MTOW in kg.
The Weight Factor is a vital part of calculating the charges the plane pays everywhere around the whole, for taking off, landing, and just flying en-route.
The only interesting thing about 10,600 lb / 4,800 kg is that the weight factor is kept a tad under 10.00 (two decimals are used in WF).
Now is there a higher charge for a plane with WF above 10.00?
I scoured the internet, there doesn't seem to be WF brackets set that way. I very badly wanted to find something. Maybe it's there. I don't know.
For big planes, there seems to be, Ryanair has a fleet specifically made with a certified MTOW of 66,990 kg (WF ~36.60) for similar reasons apparently.