It's just a combination of factors that are largely 'what if's' just now.
Star Trek Into Darkness is about ready to go. A third film will depend on how it performs. But it would probably be at least 2016 (assuming three years based on the original 2012 sequel date) before it would be released. IF Darkness is successful enough to warrant one.
The Star Wars film is dated 2015. Fine, he's completed work on Trek just now so he has time to spend making it and then it would be question of how it was recieved/performed (though being Star Wars probably flopping is near on an impossibility) would dictate plans for Episode VIII.
I don't think on a personal level it's entirely impossible for all parties to make these things work if they wan to but I think on a personal level it's probably not good for one guy to be so majorly involved in two iconic franchises simultaneously. And I mean that both for him and for the respective franchises. But then there's no point making too many assumptions just now. Too early. Plus, it's too dependent on a multitude of unknowns yet.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist