I don't think you realise just what a major step it is to offer stuff online, firstly, you need to sort out contractual agreements with Couriers (instead of just shipping Mass bulk to stores, they would now have to ship to individuals, totally different scenerios), Banks (trying to work out a decent rate for the Online Transactions, but this is my second point), suppliers, etc.
Secondly, you need to set-up a payment gateway that can deal with the transactions of various Credit Cards and Banks (This is a HUGE process, in labour and Hardware terms, you would have to expand your IT development side ALOT to develop, maintain and hopefully, move forward with this). It's not as easy as you think , not by a looooooong shot.
I'm not even going to in-depth here, there are so many issues that come up from offering Online Shopping, sometimes it just isn't worth!