It is always about the money. I remember we had a deal agreed with Juan mata but we waited to conclude the fabregas deal and avoid paying a few weeks wages. In the end Chelsea swooped him up.
The ONLY reason we are delaying with Mustafi(or any other CB target) is the same. Wenger just waits for clubs to become more desperate (or the player to grow impatient) and knock off the price a bit.
I am not getting into pro's and cons of it, but barring exceptions that is the standard modus operandi of Arsene and the club.