For two reasons we see REWs more
REW is an older color, and it's found in way more breeds & gene pools. Most the breeds that accept BEW only added it because people wanted the color; and those are mostly small breeds. Since people are breeding for color instead of type in many of those cases, BEW remains uncommon in the show world (their type just not as competitive) while being fairly common in those breeds (ex. holland lop, netherland dwarf) in the pet world. The only six class breed that comes in BEW is Beveren; which is the oldest BEW breed we have (the direct descendant of the first breed with the vienna gene, which is now extinct), and it doesn't accept REW -- all white Beverens are BEW
As the BEW gets introduced to more breed standards (just recently it was added to yet another; britannia petite) and those breeders continue to work on type, I think BEWs have a chance to get more popular in the show world. In meat breeding, though, they already have some whites; red eyes, sure, but eye color doesn't matter on a pelt so there isn't as much push to breed BEWs.
The other reason BEWs often aren't accepted is because of vienna marks (the BEW gene is the same as the vienna gene). These are unshowable, and the vienna gene can hide for generations, so people breeding some non-BEW color, if they get a vienna mark popping up, can't tell where it came from, and would have to nearly scrap their whole if they wanted it to breed true again. For this reason, only people specifically breeding BEW want BEW; with some other colors if you find a nice one, you can cross colors, but with BEWs it's not that easy. The ARBA standards committee and national breed club have to approve colors when people want to add them to the standard; and one of the things that they consider is how well the color mixes with other colors in the breed. BEW doesn't do that very well, leaving eye color problems and white markings in many of its crosses.
And about limiting colors to the standard; it takes generations of breeding to get a color consistent and good enough of type to add to the standard. It takes a minimum of 4-5 years just proving that it can show by presenting it at ARBA conventions before it can get recognized. There are plenty of people doing it, though, and if you are very dedicated to breeding a color, you could even attempt it too. For a color to be consistent enough to pass the ARBA's presentation, it means no vienna marks; they need a solid body color (no brokens allowed at this stage, even) and a consistent eye color. ARBA has said that it will not allow dutch coloring (which would, imo, include vienna), for a color COD in a non-dutch breed. In short, for a color to be able to be shown it has to pass certain quality tests, which is why many colors aren't accepted. In the breeds that aren't really color specific (like dwarfs and angoras) they are adding colors all the time; netherland dwarfs have champagne and maybe cinnamon with CODs right, english and satin angora are working on adding brokens, giant angoras were working on adding black, even. If they added every color, without having a standard for color, they couldn't know what to judge it on, and if no color had a standard many of the colors that take selective breeding beyond just the basic color to get today (like reds, tans, etc) wouldn't be as high quality, with no standard pushing them forward in that breeding. Asking why only certain colors can be shown is sort of like asking why only certain breeds can be shown, and not mixed breeds; we just only have a limited number of standards.
For a bit of history lesson here; all varieties of rabbit used to be shown differently. For example, black silvers and fawn silvers and black flemish and white flemish would all compete for separate awards and have separate standards. Variety and breed went hand in hand. When the ARBA combined those into varieties of breeds in the standard, they kept those varieties as the only ones that could be shown instead of just scrapping variety altogether.
BEWs are accepted in angoras (though the gene's newer than the standard; when the standard was written breeders sat down and just wrote all the colors they could think of to add. Angoras are an old breed, and it was done much differently then), and lionheads have been actively working on getting it accepted, btw. A new breed has a limit of 5 colors, which for lionheads was REW, black tort, blue tort, choc tort and lilac tort, and now that lionhead itself is recognized they're going through and adding the rest of the colors. So far they've added chocolate and seal.
