[notahacker]

They're a large part of some airlines' profits because there's a small number of people willing to pay silly prices on most routes at most times of day.

That doesn't translate particularly well to less frequent 55 business class seat flights on most of those routes; even if the flights are shorter the arrival times are probably a lot less convenient for most of the executives.

The all-business class model has struggled to be economically viable even on ideal routes like London-NY. Those economy seats give you a lot more flexibility when the business class seats aren't selling.

(And as ghaff pointed out business travellers /= business class; most people travelling on business have an economy ticket sized budget)