There is much more excitement for ABBA because of a number of things-Mamma Mia is well known to the current generation, ABBA has been elusive of reforming for many years and apparently turned down $1 B to reunite, ABBA had been part of the collective consciousness for a number of decades. There is much more press and publicity surrounding this long heralded return. In contrast, Diana Ross has not been absent from the public, she has never really received her due from the press and public, Mamma Mia used ABBA music but wasn't about them-Ross' fictionalized Broadway production Dreamgirls used none of their music ,wasn't the actual true story of the Supremes[[just closely resembled some parts of it), in her time Ross was named entertainer of the century but her recent music from the mid 80's on did not capture the public eye as much as her earlier work did-even comparing her work to Streisand or Aretha Franklin who continued on and had releases that were still heralded after their heyday. I am not sure if it is mismanagement, but Ross is just not promoted out there like the legend that she is. She still seems to enjoy performing and music, but it seems the strategy is quite different and it may be because she controls everything and does not have the managerial skills that lead to her initial successes. Still, both ABBA and Ross are legends but of a quite different sort and comparisons are not about their talent but public perception, tastes and the aforementioned promotions.