All different flavors of the same fruit.
The FHSS system is probably made using a cheaper-to-manufacture design (I haven't seen one so can't confirm that).
Then there's also the desire to keep the entry-level systems separate to the higher-level ones -- for purely marketing purposes.
It means you can get newcomers hooked on your entry-level product and then effectively empty their pockets again by requiring that they buy all new gear (due to the incompatibility) when they need more channels and features.
Call me a cynic but I'd say it's more of the latter than the former -- after all, the Chinese such as FrSky and Corona have shown us how much it actually costs to make a FASST receiver
RCModelReviews.com, just the facts.