I have heard this a number of times, probably more then once on this forum. Just my experience has shown slower powders to provide better accuracy.
Having started out originally as a black powder cartridge, I think they were trying to stay within the same pressure ranges/faster burn rate, maybe just so they didn't have to rework the sight graduations...? Total speculation there, but black powder being a much faster burn rate, first with Cordite then with 3031, I think they just stayed in familiar territory.
Having a much broader range of powders & burn rates now, I find the somewhat slower burn rates to work better in my rifles. My best groups with my SMLE MKIII were just under 2" at 100 yards with iron sights & 39g of IMR--4064. Bit of a cheat as the "iron sights" are a Parker Hale 5A Micrometer Apurature sight... But it's still not a scope. I did put a scope on my #4 recently and went sub MOA with loads I had made for the SMLE. Hopefully taking that to the range this weekend.