As in airedale terrier… Any idea why it's not accepted as a word?

I think the reason would be that Airedale is a town in Yorkshire associated with the terriers.

As with most word games, names of places [and in general all 'Proper Nouns'] are not allowed.

Thanks for the explanation. Much appreciated. :)

Indy has given the right answer, and what follows is pointless pedantry - but Airedale is not a town, it is a region, the dale of the river Aire, containing many towns, including Keighley and Skipton.