I had a similar issue. I was getting The Writer on my nook 1st edition. I can't any longer. It is only available for nook color or the nook tablet. I contacted B&N and The Writer with my displeasure over my subscription being discontinued due to the device I own.
The Writer did not even list B&N as a provider of a digital version of their magazine on their website - at least, that I could locate.
So, it appears that B&N is in control of digital version of magazines, deciding which devices they are suitable for. It also appears that these decisions are made without notifying customers with subscriptions. I am most displeased in the way that B&N handled that situation.
I'm not going to go out and buy a nook color or nook tablet for that one magazine subscription. I am also not sending B&N money for that subscription, which I was happily doing for months, because they made the magazine unavailable for my nook. They are slowly pushing me away as a customer.