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Popular demand + fuel mileage requirements
Popular demand is the SUV, in particular, the original Hummer as well as the Jeep. People I've talked to want "a heavy vehicle because 'it's safer'" and to be able "sit up high" in their vehicle. They also want lots of ground clearance because of potholes and snow.
But the original SUV is a terrible shape for fuel mileage. Both the general public and the EPA want better fuel mileage. Give people the option of a full fledged SUV that gets 10mpg and a smaller crossover that gets 25mpg and the majority will opt for the crossover simply for that reason. The thing is that you can't just take the original SUV shape and just throw in an efficient engine and suddenly get 25mpg or better. People also want to drive fast and the SUV shape is terribly non-aerodynamic.
So take an SUV shape and tweak it to make it aerodynamic enough to get "good" fuel mileage. That's where modern vehicle designs are coming from.
The thing about aerodynamics is that there are only so many shapes that are perfectly aerodynamic (have a Cd of 0.015) and they are pretty similar looking. The more aerodynamic a vehicle is the more it will look like one of those shapes. This is why airplane fuselages all look pretty similar.
But in trying to keep the traditional vehicle shapes alive (sedan, coupe, SUV, pickup, etc.) companies have to make some sort of compromise. They have to fit somewhere between an aerodynamic shape and a traditional shape. This has made most vehicles the same shape.
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