No inside word really but it was debated at length on the Arsenal thread.
I guess the obvious reasoning is that we already have world class options in Ozil and Ramsey to start in the two possible positions he could play, as well as plenty of players who are either up-and-comers or solid back-up for those spots. E.g. Cazorla, Oxlade-Chamberlain, Wilshere, Rosicky, Zelalem, Diaby*.
I would've loved him to come back but I guess Wenger sees 30mil as a bit steep just for a 'nice to have'. Understandable but still crushing for a fan like myself.
* just kidding