One place that it is still doing really well in is Saudi Arabia. Saudi Arabian has a rather large fleet of the planes, and uses them due to their great field performance from the less then ideal temperatures in Saudi Arabia.
-90, just like the MD
-11 was rushed into service. McDonnell Douglas cut back on the financing dollars and kept too much old stuff in the plane in order to beat the competition (737NGs or 777s). I think a bigger and newer wing and a more thorough desing process would have saved both of the MD
-90 and MD
-11. Both suffered from old technology trying to be fused together with new technology which resulted in large teeting issues for both planes, and the added bonus of not meeting the promisted objectives. If the MD
-90 could do transcons like the 738, and hadn't had the electrical problems, then it probably would have kept up with the Boeing competition. Unfortunately McD was desperate and didn't have the money to redesign planes that would successfully compete with A and B.
It is not that they didn't want to, but they didn't have the available resources or funding. Another reason goes back to the DC-8. The DC-8 had a massively long development process. When it came out it was arguable a superior plane to the 707, but lost out because it took almost 5 extra years. Boeing won that time because they beat Douglas. Douglas learned its lesson though and with the MD
-11 and MD
-90, it gave up on trying to have the best possible plane, and pushed it through design in order to make it to the market first. Unfortunately form McD, they lost again because the market had shifted away from wanting anything new, to wanting the best.
If you have never designed an airplane part before, let the real designers do the work!