Because if you do, buy it now, rather than 2 month's time.
The floods in Thailand have wiped out a lot of hard drive manufacturing - a 1Tb Samsung drive that was £40 in August is now selling for £120 on Ebuyer. This hasn't filtered through to the PC manufacturers yet, but the glut that was left after the recession bit is almost all gone, and unit prices will climb as demand exceeds supply. The cheapest external drive I could find was a Toshiba at £92: exactly like the 1Tb Tosh drive I bought 3 years ago for my Macbook costing £72.
This kind of price reversal just doesn't happen, except it has.
OTOH if you want SSD or RAM then they're both dirt cheap right now, and processors and MoBos will likely get cheaper as there is an excess compared to available HDDs for building into finished machines. I'm just hoping that my hard drives remain good for another 12 months+, and that I don't have to do a lot of system building/rebuilding for a while.
Says he, while backing up the home directory to the 'free' 1Tb Seagate drive. Now, if only house prices would halve... but that's another story.