Mostly due to installation limitations & the models offered in the lineup.
Perlisten offers on-wall models for surrounds and enclosed ceiling speakers. If I want to stay with KEF, I can wall mount an R8 Meta easily or use the R2/R6 Meta as a surround. KEF has nice in-ceiling speakers, but all are open back, so sound leakage is a concern.
We're in a high rise condo with concrete ceilings/floors. The interior has a suspended drywall ceiling which allows for in-ceiling speakers and recessed lights. In-walls won't work where I need to put them, so I need surface mount speakers. I don't want floor/stand mounts for the surrounds.
FWIW, in our last place, we had a surround setup that we enjoyed with speakers from multiple lines to make it work in the space. There I had concrete ceilings, so needed surface mount speakers for Atmos. I used Focal Domes. They aren't great, but did the trick with a very high crossover. I used Revel in-walls for surrounds and they complemented the LCR fine. At that time, I had Aerial Acoustics 7B & CC3B for LCR (now KEF Reference Meta 3 & 2).