This is a question that's been plaguing me recently.
And I think the short answer to the question has to be:-
Genre acts as a signpost, especially when the publisher chooses to put it either on the front cover or the spine of the book you're looking for. So why do publishers keep cocking it up? Or is it really not as simple as a yes or no answer?
There's nothing more annoying for a romance reader than to find out that their story doesn't come complete with a happy ending, a m/m fan to find the story going m/m/f or worse going m/f/m, or for a contemporary fan suddenly to have their favourite series go all time travel.
Is it because certain genres are now considered passe? Is it because certain genres are perceived as selling better than others? Is it because the person(s) at the publisher who decide such things haven't always got a clue?
I'm going to look at this over the next couple of weeks concentrating on Urban Fantasy.