Long story short: It's that way by design. Wig companies want you to stick with their brands.
This is why some companies use just color codes, while others use only nicknames and no color codes at all, and still others use both. It is virtually impossible to find exact matches to some colors (like Rene of Paris' Chocolate Frost, for example), while others are very easy to track down with only subtle variations between the brands (like a good old #4 or #6).
So, for an easier time of things, once you find a color you love, you're best bet is to stick within the confines of that particular brand.
I know that's no fun, but things can get very frustrating if you fall in love with a rare color that exists only in one brand (like Raquel's SS44/60 Sugared Licorice) and then spend hundreds of dollars and hours of your time only to discover that the particular color doesn't exist outside of that brand.
Only continue your pursuit of cross-brand color matching if you are 100% comfortable with a certain healthy amount of color deviation among your wig acquisitions. It's best to stay with the brand that you know works best for you if you are the kind of person for whom the thought of change leaves you somewhat unsettled.