Mainly because they want Wenger in 2014 when his Arsenal contract runs out, so they are just offering coaches one year, any coach worth his salt will more likely reject that offer.
So they got Blanc, someone who needs to prove himself again after the France debacle and not really in the position to reject a PSG job.
What this should mean though is the Ancelotti to Madrid and Higuain to Arsenal deals are tied up pretty quickly.
Blanc did a cracking job at Bordeaux like. A return to Ligue 1 is probably what he needs but don't think a short-term role will benefit him at all.