I've got a short story that might explain what happened. When I worked for Penney's decades ago, an order came down form on high to destroy all of a certain line of electronics. The stuff was good, just outdated. The pieces could not be sold, given to employees, or anything else except destroyed. The reason? There was a bigger tax deduction if the stuff was destroyed than if it was sold on a clearance sale price. So we got out some hammers from the hardware section and pounded the stuff into useless bits of trash.
What happened to all the stocks of 7800 cards that must have been produced? Can't say for sure, but some, or a lot, might have been destroyed. That way they couldn't complete against the new lines, they might have made a better tax deduction for Nvidia, etc. All that has been available may just be what Newegg, etc had on hand. After that, there is no more new replacements at any cost. Is this what happened to the 7800's? Can't say, but its possible. Tax deductions mean a lot, you know.