There are no obstacles on the roof. With the roof angled (the hatch being the highest point), a body that was slowly filling with gas and becoming buoyant could theoretically "slide" along the roof toward the highest point. There would not need to be repeated sinking/floating needed to clear the seats.
One explanation I can think of is that she didn't purposely pass up the open windows, but instead couldn't fight the pressure of the water rushing in. You need the right mindset to dive face-first into rushing water, when panic is screaming at you to get away from it. Not just that, but to dive DOWN. In pitch-black darkness. Without SEEING the window, only feeling the water rushing in, would she even register that it was a way out? I personally don't think there's any point speculating why she didn't do this or do that. In my opinion it was a death trap and she had no chance to even try any of the things we are coming up with as possible ways to escape.
Another possibility is that the passenger window was actually closed, until the car hit the bottom and rolled over, breaking it. Which would mean the rear window was the only available exit. If the doors and windows would not open due to pressure, and she could see the rear window, that's a compelling reason to climb into the backseat. Lots of assumptions there, like could she even see it or did the car nosedive so fast that there was no light available to see anything at all.
All MOO, of course.
I think if she did try to get to the cargo area it was for air, or a place in the vehicle that wasn't filled with water. I don't think it was to get out of the vehicle, because the cargo door/window wouldn't open easily from the inside, and I doubt most people would know how to uncover the part of the cargo door and then use a tool like a screw driver to maneuver the device to get the cargo door to open from the inside. Someone posted a video of this upthread, a ways back.