Because ephippiata hasn't been in cultivation for long, and none of the commercial producers have flowering size plants that I know of (although they're not far off). They will come, but I'd think that they'd not be all that flash as they wouldn't have the same appeal as lowii hybrids, other than as a curio.
from what ive seen of lowii lowers and ephippiata lowers, lowii should make better hybrids unless something funky turns up in its genes when crossed. between the two as species its a toss up, i used my hard eaned cash for ephippiata and am trading for lowii so that should tell yah something on my preferance between the two species.
Ephippiata is unique in that it seems to be much much faster than lowii, and can be rather variable in color, from purple, to reddish, to mostly green. Oh, and did I mention it's much faster, and easier? Think I could even grow this sucker as a lowlander if I had to.