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.
I already thought of this, but what's more inconvenient and detrimental to trade volume than getting shut down?