Almost every wig color in the world, with very few exceptions, has a bit of gold, neutral (or “honey”) tones, or auburn mixed in, if only in subtle amounts. Pure ash colors do exist, especially in the platinums and lighter greys, but they are definitely few and far between. This is because ash tones tend to age people, and most women want to look younger than their actual age, not the other way around.
It's also partially a matter of fashion. Pure ash tones can look a bit dated, since they were very popular in the 1980's. (Though, in many regards, this is making a comeback. [Thanks, Taylor Swift!])
Another thing to consider: these trends differ depending on where you're from. For instance, in Britain and the EU, ashy blondes are some of the best selling colors! This is why you're more likely to find more of them in European brands than American ones. ;)