Well, that's my question too. The modern approach to tensor analysis is through Cartan theory, i.e., using (differential alternating) forms and coordinate free formulations, while physicists usually use the Ricci calculus using components and upper and lower indices. I'd say, both have their advantages and disadvantages. I'd say using the mathematicians' representation free formulations has advantages as far as formal developments are concerned (e.g., there is basically only one integral theorem, the Stokes's theorem for differential forms of arbitrary rank) and also some calculational tasks are simplified (e.g., when calculating the curvature tensor in GR; see Misner, Thorne, Wheeler), while the Ricci calculus just deals with the components and thus real functions so that you can use standard calculus.