There's a tremendous amount of great stuff on Tangerine. I guess once Ray formed his own label, he recorded many of his heroes like Percy Mayfield, Louis Jordan, Jimmy Scott, Jimmy Lewis, et al, and who knows what else that only came out on singles.
Don't know if Rhino has a lock - years ago the story was that nobody was willing to pay Ray's price to license his stuff, which was why none of it was reissued. Ray entered into a deal with Rhino for a greatest hits package that apparently sold quite well, and that must have opened the door for these other projects.
As to why these are limited editions - these are sold retail on line only, which eliminates wholesalers and distributors. Every piece sold generates 20 bucks for Rhino, making it possible to release things in relatively small quantites and still turn a profit. I guess the limited status works as an incentive for sales.