I think Phil Jackson misses the competitiveness and life of the NBA. He has been a part of the league for so long. Hard to walk away from something like that.
Like he once said, he would either walk away from the game at a younger age and do something completely different and live a different life (something outside of basketball) ... or he'd stay in the game and become one of those coaches who spends their entire life around basketball and never does anything else.
I think he just spent so long in the league and is so competitive and misses many things about basketball, that he wanted to come back. But his health is no longer good enough to coach full time anymore so he went for a front office position instead. If it wasn't New York, it would be somewhere else.
New York was just the team (a place he has always said he feels a special connection to from his time as a player) that offered him the job he wanted, with the control he wanted and the relaxed duties/responsibilities (in terms of day-to-day GM stuff) he wanted and then threw in a huge payday to boot. It was the perfect offer for Phil Jackson.