in fairness to Fred and Ginger, they weren't just famous for dancing, their films were romances as well, in which they ended up together; their dancing reflected the (on-screen) relationship. The Nicholas Brothers' dancing obviously didn't have that kind of emotional undertone - it's done for fun, not for passion.
Fred is better known than Ginger, but putting the two of them together meant combining his grace with her sassiness - she'd previously been more of a comedienne. Someone said 'He gave her class, she gave him sex.' (Not literally - it meant she made him look sexy, which he didn't otherwise.)
And as Ginger allegedly said: 'I did everything Fred did, but backwards and in high heels.'