Well, the diamond types are a fairly recent idea, so some of it has to do with tradition, it's how fine goods were done in Europe, marked with the pricking iron, or wheel at a high SPI then the hole made with an awl.
You won't get near the same SPI in diamond chisels, around 8/9 SPI is about as fine a stitch as they will do, thats about the starting point for most euros, it's pretty chunky for Euro tastes, although has become a bit more common, around 9 is common on bags, higher for wallets etc It's not uncommon to see 10 or 12 SPI over here, some even go to 14 SPI
also, the diamond chisel tends to distort the leather a bit too much, they punch pretty big holes compared to using a pricking iron/awl
As for cost...
Well the Blanchards are made in France so that explains a large part of it (very expensive country), also, they tend to be for pro's/businesses so are priced accordingly.
Don't forget too, they are made by a craftsman, no machine milling, they are ground & filed by hand, not an easy job to do well, especially high SPI irons.