Why was the DC-9-80/MD-80 so successful despite being obsolete almost from birth?
It's simple.  Cheap (development costs amortized decades before) and reliable.  They made money for airlines.  Or, you know, they wouldn't have bought them.  There's more to operating costs than fuel burn, and in any case, fuel prices in the mid 90s were cheaper than at any time since the 20s in constant dollars.  Fuel was so cheap that Air Canada was bringing L1011s out of mothballs in the mid 90s because they could operate them Toronto to LA profitably.