its because people are still generally very short sighted in plans, unless your well established with no debt, you generally are placing some risk on anything that takes over 1 year to break even
at 1 pound per watt, your talking 1000 pounds per KW, at at average power costs, and ~8 hours of usable sunlight, that still takes 3-4 years to pay for itself (excluding feed-in tarrifs)
the other issue is, during the middle of the day, average joe's power draw is a fridge or 2 and a hot water system if not on a timer, plus a sum of under 100W standby from other things, so its mostly just being fed into the grid,
if he does store it, there is the additional cost of batteries, and replacing and maintaining those batteries over the years, plus the inverter, which cost quite a bit
on top of all of this, you still have the issue of the panels requiring more power to make than they generate in 5-10 years, these are why they still arent all that popular and booming on larger scales, though with the whole carbon pricing thing over here, it may just force peoples hands into installing them
the other often overlooked issue it, when everyones systems are trying to feed into the system, it pulls the line voltage up to the maximum of the specification, midday i commonly see it fringing on 254-260V, which none of the cheap chinese crap thats made for us 220V and resold here appreciate, with the input capacitors being the biggest downfal in almost all the chargers i have to deal with, and input diodes on some aswell (370V AC peaks easily exceeding 400V on spikes)