A brief explanation why…
IBM's Deep Blue system defeated its first world chess champion on February 10, 1996, when Garry Kasparov lost the first game of a six-game match (he won three and drew the other two games). More than a year later in May 1997, Kasparov narrowly lost his six-game rematch against an upgraded version of the machine and went as far as accusing IBM of cheating, which denied the claims, refused a follow-up rematch and retired the system.
Development for Deep Blue began in 1985 with the ChipTest project at Carnegie Mellon University and the system relied largely on brute force computing power to play chess. It was a massively parallel, RS/6000 SP Thin P2SC-based system with 30 nodes, each containing a 120MHz P2SC microprocessor enhanced with 480 special purpose VLSI chess chips. Deep Blue's chess-playing program was written in C, ran under the AIX operating system.
During its rematch with Kasparov in 1997, Deep Blue was capable of evaluating 200 million positions per second, twice as fast as the 1996 version, and was the world’s 259th most powerful supercomputer.