Most riders interested in racing know that several time trials offer a Merckx division to ride ones road bike. At the Fiesta time trials, they have a Merckx division and then a conforming Merckx division where one rides a road bike similar to Merckx. Not many sign up for riding their road bike. We may get a couple out of a hundred entries.
Generally, racers, who ride the aero bikes, often ride a second time trial on their road bikes. In the Beat the Clock time trials in NorCal, they had a Merckx division and teams. I would do a 20K ITT aero and then a 20K TTT.
We had an evening time trial on the same course as Beat the Clock and racers showed up and raced whatever they had but mostly aero bikes.
Upshot...not much uptake for riding the road bike in a TT unless one is a trained time trialist. Hell, we could offer ebike time trials and once the ebike’s power was tapped out and the racer had to put in full human power for 20 Km, no one would show up.
Timed events are just too hard and demanding for most racers or they just do not want to put in the training necessary. So be it.