Why would a diverted airline flight return to its departure airport rather than land at the nearest one?
It sounds like it was a non-emergency condition where it wasn't feasible to continue to destination but it wasn't imperative to land ASAP. Something like an anti-icing malfunction where you can't continue into icing conditions (all you need is visible moisture and temps close to freezing; clouds of any kind near or above the freezing level constitutes "technical icing conditions" and you may at minimum have to have working cowl anti-ice to fly through them).

The crew will often consult with airline Operations and Maintenance Control over a desired diversion airport.  Having decided not to proceed to destination, the next best choice is to go back to the departure airport, where passengers that originated at the departure airport can simply go home and return for a later flight.  

The other major factor is maintenance related.  Maintenance Control will often ask the capt to divert to airport X because the resources are there, so if the departure airport was a maintenance base, that's another plus.