Few reasons,
1. don't need 7 seats
2. The F25 X3 is the same size as the older X5 and fits 5 adults comfortably.
3. Cost. As you'd only want the 2014 onwards X5 to compare with the F25.
Next motor will be the new X5. We get a new X3 G01 I think in 2017...
Never liked the old original X3, the F25 is a whole different animal. And very progressive on the road. Plus residuals are really strong, which is nice :)