yeah that might be it. plus since a lot of them have the reverb/tremolo channels i'm guessing that makes them a lot more expensive, which presumably keeps demand down a bit too.
Yes, the reverb will add quite a lot in parts cost.
The BF tremolo uses an optocoupler comprising of a neon bulb and an LDR which contains cadmium sulphide and is thus not RoHS compliant, so you can't really use this in a production amplifier.
The Fender BF RIs now have a link to a PCB which substitutes for the optocoupler (although they still have the space on the board for the optocoupler).
Perhaps they still use the optocoupler in the US, although California have implemented their own RoHS regime.
Alternatively, you could have the optocoupler retro fitted, which is entirely permissible.