It's a very logical thing that SO MANY people around the world love ABBA. If you analize the music, you can say that every aspect within music production is present on a very high professional level.
That means the composition, arrangments, the use of vocal hooks and lyrics, the way it was produced and mixed.
There are only few other bands or artists on earth who were able to do similar things. (Pink Floyd, U2, Madonna...)
But Abba is somewhat different in that way that they were more mainstream so acceptable for much more people around the world. Even today ABBA stands and it will stand in the future.

Hi I am from India. I love ABBA. They are the best thing that has happened to the music world and i dont think anyone can come close to their style of music. Their music really touched my soul. Though their unfortunate split, I wished they were together even now. I just love them and their music. I am short of words how to describe my feeling of joy. I would also like to let ABBA know if they happen to read this post, that they have considerable fan following in India. LUV u ALWAYS ABBA