It is so rare because tritium is not found in nature in any significant quantity. The amount on the market comes from water recovered from water pools used for spent nuclear material storage.

There also is no enormous market for tritium. So in short, fusion reactors exist on both sides of the tritium market, by becoming the primary producers and consumers of it, which should lead to significant drop in price of tritium.

Current demand is a couple hundred grams per year. We just don't need that much of it. The cost per gram would go down a lot if we needed to mass-produce it.