Yes to a certain extent it was about backlash against another case of more "radio friendly" "extreme" music taking over the mainstream and being called "metal." At the same time, a big reason why many metalheads continue not to include nu-metal within the metal genre is that musically it tends toward being its own sub-set of rock. When you talk with the most dedicated of metalheads what is it that we very often tend to praise the most? The riffs, riffs and more riffs. Nu metal in generally tends to not be as focused on riffs and riff progression as the average metal band. Granted I haven't listened to any Slipknot beyond their first two albums and even then its been quite some time. But in those cases the guitarists role was restricted mainly supporting the vocalist. In fact, everything seemed to revolve around the vocalist. It wasn't about musicianship or addictive, memorable riffs or solos. That seems to be a trend for many nu metal bands, that the guitarist plays more of a supporting role than being able to stand on his own.
Also wanted to add that its a bit different to say that nu metal is underrated than to say that it isn't considered metal by metalheads. Like others have said many nu metal bands have much more mainstream success than many metal bands without necessarily being as good. So for many metalheads who don't like the genre, its can easily be seen as overrated.