There simply isn't the need in SF for double-deckers, as there isn't traffic:
Gridlock? Congestion? Bedlam? No. (I'm not saying HK has that, it just has more traffic, which surprisingly to somebody who lives in North America, comprises of mostly buses, not cars
)
I'm also not saying we haven't thought about it:
It's an Enviro 500! We were testing it for about a month, then had to return it to Las Vegas
The main reason for considering this bus (as a replacement for the Neoplan on the Enviro's left) was
less space taken in the garage. There was only a
mention about the increased capacity.
But still, this was the best thing SF has done in, well, a long time.
Edit:
However, I do agree that the articulated buses are not suitable for routes travelling in older areas of towns in Europe and hilly cities and they do load and unload faster, simply coz theyve more doors.
But the driver seemed to be far away from the end of the bus. Wouldn't it be difficult to spot people jumping into the bus at the station, esp. during peak hours? 
If San Francisco isn't hilly, I don't know what city is... But the biggest problem with articulated buses (and all other buses) in SF is that fare evasion is a HUGE problem, especially in Chinatown. (It's all done in the open! People aren't trying to hide it! Just walk calmly past the driver, or jam through the rear doors!)
