I think part of the real answer here is that Disney is going "all in" on the whole "effective mask" concept. At the waterparks, masks are obviously not compatible, and so, Disney is choosing not to operate them.
I also think, as has been already been mentioned, there are numerous other complicating factors, including that the lifeguards are primarily CP/IP (not there now).
You (plural "you," nothing personal) put all this in combination of Disney asking the question if it's worth operating, and apparently, so far, TDO's answer is no.