It's been about twenty years of flats for me too.
Two factors among others in the increase in use of flats are:
1) The internet has allowed the growth of bass forums, such as Basschat and Talkbass, where strings get discussed and people are encouraged to try new things e.g. flats.
2) The growth of the internet has also led to great and easy availability of a wide range of strings, ready to be purchased at the press of a button, whereas the old shops stocked a very limited range of strings and mostly not flats.
Some might want to call it a fad, but flats are not going away.