Because an X3 has a higher driving position, which gives you a better view over the road. Also because the BMW 3 series is very low, it is terrible to get in and out. Some people , like me, have some troubles with their back (not everybody here is 21 you know...
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The X3 is more capable of being driven on roads that are not level, without the road scraping against and likely damaging the chassis and underbody.
4x4 is not always needed (however I would always go for it.)