Please explain your examples... I don't get it yet.
NW 757... an oddball, OK, because NW always ignored the 737 and 767, but the 40 757s flying right now are the only aircraft filling the gap between their 150-seaters and their big DC-10s and 747s, which makes it perfect for their longer transcon flights, or long thin routes like MSP-ANC.
The Delta 772 is a good replacement for their Tristars and MD-11s. The 767 is a bit to small for their longest and most prestigious routes, and I imagine it's marketing-wise to fly a well-liked big aircraft. Imagine if DLs biggest aircraft was the 767 (if the Tristars and MD-11s are retired), DL would be taken less seriously by customers and press, and nobody would believe they're almost the biggest airline in the world. Of course economics is the main factor... therefore DL (and AA) don't buy 744s, but the 772 is able to perfom fine economically on their route-system.
Best examples of airliners which doesn't fit in, are the ones bought for political reasons. BOAC and BEA (before becoming BA) were stuck with British built aircraft which they didn't want. In the cold war, airlines had to buy aircraft of their alleys, so many (ex) communist countries got Tupolevs while they preferred Boeing/Airbus.
Or sometimes an airline isn't happy with an airliner, which was originally planned to replace half or whole their fleet in that range, and ends up with a small number, along other comparible types. The 15 MD-90s of Delta are an example, which operate alongside older 722s and MD-80s and newer 738s, all with about the same capacities. Or the MD-11s of Delta and American (they originally planned to get 40 each of them, I guess, but they stopped after 15/18).
nobody has ever died from hard work, but why take the risk?