In general: Because you're trying to do business, and doing business as a hidden service will immediately kill most of that business.
What would happen to the trade volume of ebay if ebay.com suddenly moved to ebay(...).onion?
Even SilkRoad would (I presume) explode with traffic if it was feasible to operate securely in the clear. (Which it is not.)
Going hidden has to be an strict necessity in order to outweigh the not-so-trivial inconvenience to the customer.
(And the resulting loss of business to the business.)
Update: Having an optional hidden service would be nice, novel, and likely to attract some modest additional business, without being too costly.
Low hosting fees on hidden services, innit?