The Giro is a fundamentally different race than the TDF. There is little emphasis placed on time trials and there are no real "high" mountains as in the TDF. So the TDF demands more from it's victor than the Giro. To win the TDF, you must be a good time trialer and climber, a more rounded rider. Some would cite exceptions like Pantani, but many questions surround his performances. Look at Simoni, Garzelli, Gotti, Hampsten, Salvodelli, Casagrande, Bugno, on and on. Look at the top three in the Giro and analyze where they finish in the TDF. Generally, they are nowhere. Sure there are exceptions but they are exceptions, not the rule.
It would be interesting to do an analysis of some top ten finishers in the Giro and see where they finished in the TDF in years they did not ride the Giro.