Because R2R done right tends to be allot more fluid, clear, and analog sounding. Think of your very sensitive very revealing speakers as a microscope. When you have that in your system, the rest needs to be right or they will tell you all about it. I will say though, that the most important thing to get right with a sensitive speaker is an amplifier that makes them happy.
And WIlson, and Evolution Acoustics, and many others
High-efficiency doesn't guarantee good sound quality, that's all. There are many ways to skin that particular cat, and not all of them lead to (to my ears) good results.
Probably they like R2R because the new ones are more discrete in design, which means that the digital filtering can be carefully selected or non-existent. Digital filtering is usually the culprit in Delta-Sigma designs. Orthogonal to high-efficiency speakers IME.