It's that they are basically domestic Toyotas, because Toyota and Gm partnered with the making of certain cars. Prism gsi cars are not that common and therefore a rare commodity.

the quote below is from this article hope this helps.

"There are later versions of the 4A-GE (small port high-compression, AKA red hat) that don't use T-VIS and are more powerful, but they are harder to find as they are only in 1991+ (???) Corolla GT-S's and later Geo Prism GSi's in this country"