The crossplay is not something that devs or even publishers can decide about it... So, it is more a problem of economics and profit than programming.
...but there are a few complications... for example controls. PC players have a very clear advantage.
Hey guys,
As the OP eluded to, this has already be solved by other games.
First, most cross play enabled games allow you to choose who you play with - same platform or all platforms. This is how they got around people worrying about one platform having an unfair advantage.
That said, the XBOX One now officially supports Mouse and Keyboard, and has a Hotas option, so really it has the same controls as a PC.
Next, cross play between PC and XBOX can be implemented by ANY developer as long as they follow the guidelines like Fortnight, Rocket League, and so many other games have done.
So Frontier could implement cross play if they wanted to, but I'm sure doing so would cost development time which they probably don't want to spend on cross play.
But if they did implement cross play with the XBOX, that would give them a good case to also implement it with PS4.
Personally, I really wish they would implement it!
o7