I don't think it is their choice. I think it is just because it was just being built that way by Wrightbus.
In any case, it would make little sense for a straight staircasebecause it means there will be only 2 seats at the low floor area,pretty poor should I say.
Be careful you say this. Some folks will waste no time claiming that Citybus and KMB want the E400 so badly that they probably paid ADL to build it, whilst the B9/Gemini is so unwanted that Volvo have to give them away!
Seriously though, I think both the E400 and B9 were built on request by KMB and Citybus, rather than being pushed by the manufacturer. It seems quite clear that, to me anyway, ADL actually pushes the E400 more proactively (Rumours on the E400 for HK, from my knowledge, actually dates way back at the turn of 2008/09). With that, Volvo obviously got asked, or got wind themselves, and go away and build a 2-axle version for both operators. People can argue all day about who is more reactive and who is more proactive, but I say God bless competition!
peterpan/syp: I firmly believe the reason for the 2-axle B9 to lose so much low floor space is they have fitted the fuel tanks, quite possibly all of them, just forward of the offside rear axle. The B9s in the UK have the fuel tank fitted just above (and forward) of the offside front axle. Even with the optional larger fuel tank, I have not seen any B9 losing so much low floor space like the Citybus/KMB ones do. I guess the HK ones have this features so that they can use the high pressure fuel pumps, but it's such a shame because the B9 could, and should offer just as much low floor space as the E400, as illustrated in these pictures:
