Ummm, because it's a classic combination, and for good reason! I use Tri-X processed in ID-11 or D-76 1:1 almost exclusively these days. The sharpness is very good, but not harshly so (yes, for the kind of work I do, there is such a thing as too sharp). The grain is definitely present, but is soft and beautiful. It yields a wonderful glow when properly printed.Why this combination. D-76 is high in sod sulfite and the results won't be as sharp as a higher acutence developer.
Of course, the 'best' film/developer/paper combo is entirely dependant on how and what you shoot, and your personal preferences.