I think it is because they don't want Elite Dangerous to become another First Person Shooter clone, they want ED to be different.Never really understood why people who are presumably Ok with shooting the crepe out of spacecraft have such a downer about shooting people who aren't in spacecraft.
Also, people are getting the abbreviations confused, the expansion isn't FPS it's EVA (Extra Vehicular Activity) unless I missed a memo somewhere.
And yes, it's long been speculated that development on The Outsider would be used in Elite Dangerous, but only Frontier can know for sure and if they don't want to comment on it, they won't.