The GT-S is a tiny slice of Celica sales numbers, and the tC is way cheaper than even a stripped out GT which only exists in theory because Toyota always builds em with a bunch of options tacked on.
The tC gets you power everything, ABS, 160 hp, 17" wheels, the cool glass roof, nice stereo, and some other goodies, for $16,465. The only factory options are an automatic for $800 and side airbags for $650. A comparably equipped Celica GT manual stickers at $20,270.
When the two cars are in the same showroom, how many people do you think are really going to pop a couple grand more (assuming they're smart and get a discount) for the Celica?
edit: A GT-S 6-speed with the minimum level of options that Toyota tacks on at the factory stickers at $25,710.