I'm a Roux solver, and I think there are some legitimate reasons why cube preference seems to shift between Roux and CFOP.
1. Movegroups
I think that because the most common move groups are so different, there are reasons for people to use different cubes. CFOP primarily uses RUFD moves, mostly in F2L and LL. Meanwhile, Roux uses RrUM and MU for large portions of the solve. I think that RUFD generally requires less stability than RrMU/MU. As you are turning only one layer at a time and can keep the cube mostly stable with the other hand, you don't need a super stable cube. but for RrMU/MU - where you frequently have only part of one hand on the cube or are in a somewhat awkward two-handed grip - stability becomes more important. If the cube is flopping around during SB/LSE, you run the risk of locking up.
2. TPS
This is somewhat related to move group, but also to the types of turning most beneficial to each method. The focus on algs and extremely fast execution/lookahead with a flexible move group in CFOP can be benefited by more flexible cubes. The extra corner-cutting allows cubers to spam algs and F2L incredibly quickly. Roux on the other hand tends to take a slower approach to TPS, at least for F2B. We need to see what's going on so we can solve our blocks efficiently and track pieces through blind spots. Therefore, we don't really need that minor TPS boost that you would get from a more flexible cube
TL;DR - Roux needs stability for F2B/LSE and de-emphasizes the speed of cubes, while CFOP benefits from flexibility and speed.