Prehaps if you would of played the superior version of Code Veronica (See: DC) - you\'d realize how great it truly was. It took the series in the world of polygons and what not. It wasn\'t as much as an evolution as RE3 or RE0. Both of those games brought signifcant changes to the series, some for the better and some for the worse. Either way, the series has changed. Infact, if you look at each Resident Evil game , you will see how each one had a new feature or evoled the gameplay slightly. Capcom always takes their series and tinkers them to perfection. They done this with Street Fighter also .
I personally don\'t care for RE0\'s changes - but there is no denying that Capcom does take their time and make the series fresher.
Furthermore, the point of a sequel is to maintain the same style of gameplay (for the most part) and simply improve upon the formula that was previously succesful. Otherwise you would alienate a complete audience who bought the sequel hopin\' for more of the same.
Silent Hill on the other hand, like Bobby said, hasn\'t changed any in the sequels.