I also can't recall the Adria flights having been advertised in the UK
Neither can I, but there have been a couple of newspaper articles publicising the route, of which this is one:
Maribor, Slovenia: let?s hear it for the grapevine | Travel | The Guardian
From my occasional presence when the Adria flights arrive I would say that the load factors look very encouraging. I also agree that the current timings are hopeless for UK travellers. The article above makes Maribor sound an interesting enough destination for a visit - but perhaps only once!