I started buying xrcd's about a year ago and wondered the same thing. I called JVC America and talked to a high level techie who knew a lot about the process, but didn't know anything about other labels using the 20bit k2 process. I did my own research and this is what I discovered: the core of the xrcd process is the 20 (or now, 24 bit) encoding process, but that is only part of it. JVC brags that they use the best of technology at every stage of the process, from analog master to burning the end product. Probably most important, in addition to the K2 process itself, is the zillion dollar Sony equipment they use for the conversion from 20 to 16 bits which keeps jitter to a minimum. Although other labels use k2 (such as prestige), only XRCD claims to pay such close attention to all phases of the production.
I also did have the opportunity to a/b 2 or 3 titles (on my Linn Ikemi)which were available on both XRCD and Prestige's K2 process and I admit I couldn't hear much of a difference - both sounded great. But if I had to choose between XRCD and other K2 process without the opportunity to compare, I would choose XRCD simply because I have about 12 XRCD titles and not one of them is less than fabulous.
My favs: Bill Evans - Sunday at the Village Vanguard and John Coltrane - Black Pearl
Enjoy the music!