Here are my thoughts:
Taylor Swift always seems to release when no other major pop girl has an album out, so that alone attracts consumers into checking out her album because it's new and fresh at the time, and there's no other big name to compete with. I sincerely think this is a huge reason why 1989 did as well as it has. The main popstars have been MIA for awhile, and 2014 was filled with one-hit wonders and debut artists so her release was very hyped and anticipated. 2015 continued the trend of one-hit wonders and new, irrelevant artists, so it wasn't very hard for her to dominate, especially with the lack of females on the charts this year.
I'm definitely a casual fan of Taylor Swift, but I have to say that I do not care for 1989 that much, and Blank Space is the only track I find myself listening too because it still captures the essence of her signature writing style that I really enjoyed. Her country-pop music is far better than her pure pop music, and in all honesty, I think I like Katy Perry's pure pop music more than hers.
Taylor's is too safe to the point where it's really uneventful and uninspired to me, but that's appealing to a lot of the general public, especially her target demographic (11-16 year old girls). She always plays it safe, she never takes any risks, but it works for her. Since her formula has proved successful in the past, I honestly doubt she'll change it until she starts declining in sales.
She also has a GREAT team behind her, who constantly removes any YouTube videos of her 1989 songs, the album isn't on Spotify, and it's honestly very hard to get a listen to the album unless you preview it on iTunes, pay for a streaming service, pirate it, or flat out just buy it. I think the casual listener probably thought, "Hey, this is doing well, I'm just gonna buy it because everyone else seems to like it". It's a bandwagon kind of thing. I don't think it will be one of her more remembered albums in hindsight, at least in the US anyway. The Fearless era was far bigger than this one, despite the fact she might not have been the #1 artist in the world at that time. The music industry was bigger and more of an event as well back in that time.
So I would say, it's a mix of dedicated fans, hype, and strategic marketing that helped the album do as well as it did.
Chart success & Sales Success = ATRL Acclaim.
