There are a couple of major factors associated with why Air Asia did NOT launch MAN and why TK MAY expand MAN operations.
Firstly, and most obviously, TK already fly from MAN, they are 9 hours flight time nearer to MAN than Air Asia's new KUL long-haul base.
Secondly, at the time of Air Asia's deal with MUFC, there was no long haul aircraft to be seen. They made the correct decision to launch STN over MAN yet I would never suggest that MAN would fall off of their radar as it would be a great route, but not as lucrative as STN and you cannot launch two destinations into the same country when there is already competition as fierce as EK, QR, EY, LH, AF etc.
THIRDLY, the route IST-MAN-LAX was mentioned some time back.
It may be the case that TK have made their deal solely for advertisement, however they have NOT signed a shirt deal - their logo will not be seen around the globe week in week out as has AIG for the past X amount of years. It may be simply for the advertisement boards and programmes but I think it is something more than that...