It's both an aesthetic and a functional thing. Most ABDLs prefer the plastic-backed because all baby diapers used to be plastic-backed. Maybe 20 years from now when there's a new generation of ABDLs who used today's cloth-like baby diapers there will be a demand for cloth-like ABDL nappies. I've found that there's a similar generational preference difference where older ABDLs (those born before the advent of disposable diapers) often prefer cloth nappies and plastic pants.
Functionally, most people with severe/full incontinence will attest that almost all fabric-backed nappies are inferior to plastic-backed nappies in terms of leak protection, odor protection, absorbency, and durability. There are a few exceptions, like Seni, which come close to matching their plastic-backed rivals but most brands still have a long way to go.