Well, Toomey gave the "spend more time with the family" reason, but I think there may be another reason or reasons. He barely won re-election last time and maybe he knows he isn't all that popular these days. Certainly, his bipartisan "common sense" gun control legislation attempt with Joe Manchin soured a number of voters on him. Perhaps, Toomey realized he likely would not win a third term.
Toomey was also mentioned as a possible candidate for PA governor, but he has now nixed that idea too. The last Republican governor, Corbett, tuned out to be more like a tax and spend Democrat, so I'm not so sure Republicans have a chance of winning back the governorship in this state anytime soon. Maybe Toomey realizes this and didn't think he could win.
In any event, I have always favored term limits in both houses of congress, so I'm glad Toomey is leaving after two terms. Time to bring in someone new.