He felt that the engineers intervened DURING the games, and obviously did not feel that intervention between games was reasonable either. The former was disallowed, and the latter was taken advantage of more than Kasparov thought it would be.
He wasn't being a sore loser and there was a reasonable amount of agreement among other chess professionals (engine authors and players) that his claims were true.
Keep in mind as well that when he asked for a rematch with more stringent scrutiny in place, IBM declined and nearly immediately dismantled Deep Blue.
The unfortunate thing is that kasparov would ALWAYS have looked like a 'sore loser' unless IBM decided to implicate themselves of wrongdoing. Kasparov being silent also does a disservice to the community, particularly since he was neither the first or only person to point out the inconsistencies in Deep Blue's play.
Keep in mind as well that the 'real circumstances' make it so both parties responses were rationally founded within the context of the available information at the time.