That's doesn't have anything to do with the scenery, that have all the runways in places, but it's a decision made by FSX using several factors (runway length, wind direction/speed, ils avail, etc.), each one with its own weight, with the wind having the most effect.
You'll always get the longest runway with an ILS, provided the tailwind component is not too strong to require a change of runway. That's how FSX always worked. Of course, I just checked, with strong winds (24 kt) from 306, it gave me the runways 33s, but with wind from 180, it gave me only the 15s.