Because for the extra £50 you get 2 games (worth more than £50 anyway, or at least £30 sold on eBay afterwards and going on some sales you might make back that £50), 20 GB extra memory and the extra USB ports, SD card reader and PS2 compatibility.
BTW - I just got a PS3 60GB from Empire Direct. Try them as they were getting some more in stock!
I tried everywhere else and they're all sold out! Because of this even 2nd hand 60GB PS3s are hitting the £400 mark on eBay.