From my position here, the reasons you don't see as many radiant cuts is simple- money.
As it stands right now- and has been this way for the better part of a year- round diamond prices are so high, that Fancy Shapes have become undervalued by comparison.
Therefore any cutter with a choice, is choosing rounds.
The solution for this will come as more consumers demand fancy shapes, and the prices of them will rise- prompting more cutters to cut them.
Then we come to the many variations on the design created by Henry Grossbard in 1977 ( sorry for giving your age away Squig:)
In many cases, cutters have gone in different directions than Henry, creating stones that look more someone had pasted the facet pattern of a round onto the shape of a radiant. Many of these variants are quite unattractive to my eye.
OMC described it nicely. When done correctly a radiant tends to grab light from a wider dispersion- and return it in a different way than a round.
I disagree that the longer light paths make it a bad choice for colorless- have a look at this one- an Original Radiant Cut, in a D color- it compares favorably with a ideal cut RBC for "whiteness"