My wild guess would have been aerodynamic drag had something to do with it.
Larry.. if it was on the lemans cars.. sure i'd buy into that.. and i assume thats why they ditched the peaked "eyebrowed" rims on the lemans tigers, alpines and the circuit cars in many cases. But they didnt use the rootes parts, just the plain round ones.
On the rally cars they used the rootes ones.. as i said, they stop the ridge that things might build up on... but there would be no aero advantage as they just dont get to the speeds that it would make a difference.
It was a consisient choice. and in rootes fashion an odd one.. its not like they didnt have the parts in stock.. they were not rare then, just off the shelf items.. it predates the MKII itmes (they used chrome rims on the tigers and alpines, the husky, singer gazelle items.. not the painted ones of the mKII)
If aero was the true aim.. they really hsould have used the rootes items on the LM cars.. which some of them now have.
I have also heard a suggestion the race cars used the plain round ones as they may have been clip on and faster to change incase of breakage.. and the rootes clip ons had a habbit of clipping off...
probably the reason is lost to time.. but they were consistent in the factory use of the roote sones on the rally cars on snowy events (and a couple of others) and plain round ones on factory track cars.. sebing alpines, LM alpines 61-63 and the LM tiger in 64.