Hi Dragon,
I don't have any detailed pics on me now(i'm at work pretending to look busy) but it looks to me like the reason is the height of the upper deck, and the shape of the Mk.2's deck near the rear of the car.
The Kyosho's upper deck seems to be one flat piece, which is too low to accomodate a hump-pack, the result is the lay-down/stick-pack taking up more real-estate under the deck, so there's not enough space for the RX. I've never seen an FW05 up close to compare, so I'm not quite sure but here's a pic:
As for the Mk.2, the upper deck isn't flat. Just behind the switch it gets slightly wider, and is higher as well, just enough to squeeze in the hump-pack and RX too. It's actually not sealed though, just an upside-down tray with a lip. There's not much space left for servo/battery/switch wires to be tucked away though. This pic shows the shape pretty well:
I'll try to get some more detailed pics up for you.
sml_blok
--------
TG10 Mk.2
Savage SS 4.6