The reason there's not a ascii95 is because 85^5 (5 bytes of encoded string) is only a little bigger than 256^4 (4 bytes of unencoded string), which makes it a particularly effective encoding for blocks of such a small size. In fact, you have to get up to 21 bytes before ascii95 would be an improvement on ascii85 (21 unencoded bytes -> 26 ascii95, but 27 ascii85). By that point, the numbers are rather too large to deal with reasonably - we're barely at 64-bit computers, let alone 168 bits!
Since ascii95 wouldn't really be a feasible improvement on ascii85, we may as well just use 85 and then have those 10 characters for whatever we want, like 'z' and '~'.
(Plus we don't want to use space, so it would really only be ascii94. That doesn't change the math, though.)