I thinks sometimes developers or their graphic people don't think about things from an earlier game having that much impact on the new one. Seems like alot of times they just look for ways to cut cost.
Actually, I don't think it's just about cutting costs but also about laziness and wanting to pander to the so-called "mainstream" gaming market. Why laziness? 'Cos good game design can be rather challenging and tough to implement, whereas sub-par game design is a lot easier to envision and implement. After all, if you think of a lot of innovative ideas, you also need to take into consideration various factors and weigh them carefully, especially when you've a budget and limited resources. And then, you also need to ensure that the ideas flow and click together.
On the other hand, creating a lot of shiny graphics means you'd need a rather huge budget: license for programs, license for 3d engine, 3d plugins, game engine, etc. I'm sure they're reusing some of the stuff they did for Oblivion, however, creating the rest of the "beautiful" explosions surely requires quite a bit of a budget.