Because I can fit 4-5 inlays into one pressing with a small ingot, then my ingot price is down to $4 or $5 per unit. Not rocket science, but because they have so much volume coming in it is much easier to maximize their ingot usage, along with all the other material costs associated with it.
My workhorse ingot is the LT with a little more now going to the value shades (V1, V2, V3). The issue with the HTs is that they were developed to be an inlay, onlay and veneer material. Because of this, they do great with thin resorations (<1mm). As the HTs get thicker (especialy >1.5mm) they tend to go grey due to their translucency and the darkness of the oral cavity. SOMETIMES, you can get away with going a shade or two lighter/brighter to couneract this effect, but not always.