...because

a)Neither sounded like the stuff that was typical of the time
b)They both got namechecked as an influence by cool and hip artists later on.
That's pretty much what "ahead of their time" boils down to; Frank Zappa and the Mothers of Invention also didn't sound like what was typcal at the time but they "never" (well, after the late 60's/early 70's) inspired many cool or hip n¨bands-mostly just fusion- and prog-nerds with ponytails and glasses. Hence, they are not viewed as ahead of their time or "timeless" as velvet Underground.

Basically, it's not enough to sound atypical and be influential, you need to inspire the "correct" stuff as well...