Possibly because it was designed years before LO was even thought of. I thought the purpose of the longer northbound platform was to allow for signal positioning for reversals back in a southbound direction, but that's a bit of a hazy memory now. It might also be because knowing the platform was much narrower than the southbound, trains would normally operate from the part of the platform well beyond the lift and stairs?
In the months before it first opened it was even kitted out with Silverlink style name boards as per the contract, as it had been on the go for that long.
http://en.wikipedia.org/wiki/File:Shepherd's_Bush_stn_Overground_signage_pre-open_Silverlink_07.JPG
IIRC it had also appeared on the 'London Connections' map in about 2005, marked 'about to open'.
By the time it did open, they were seriously concerned about the space available on the northbound platform, and that was rebuilt at the last minute, delaying the opening. The background to that problem was that the street design above had needed all the available width because it had allowed for a tram terminus.
TfL tried to spin it as a design error, but that was not quite true...
