Because there is no benefit to be had from selecting 1.6 over 1.4. 1.6 was an emergency update primarily released to overcome the battery shutdown issue introduced in 1.5. 1.4 & below do not suffer the battery shutdown issue. The "solution" offered is to limit the Phantom's thrust under certain situations, thereby reducing power & performance. At the same time DJI began implementing the GEO feature. If running 1.4 there are no new features or benefits to running 1.6. Further, once you go to 1.6 you are committed & can't go back to 1.4.
