There are three problems to explore. Guyver is very.....very.....very generic. When I say generic....there is nothing more generic than a high school boy throwing on a suit and beating monsters out of their misery.
Guyver is underrated because it is lacking. The story is always pretty much the same. It starts out....
Sho becomes the Guyver
Sho learns how to use the Guyver
Sho learns how to kill zoanoids
Sho kills zoanoids
Sho gets another power boost aka The Gigantic
Sho goes on to kill more zoanoids
So on and so forth. The series really doesn't offer anything new to anyone. If anything it probably has a hard time competing with superior anime series and not just that.
The major thing is with any anime/ manga or Guyver is that, both have Marvel and DC to deal with in the West. Not just the comics, but movies and shows as well. As time goes on, American TV continues to improve.
Another primary thing is.....Guyver lacks DEMAND.
Why? Because there are better stories out there. Does this means the Guyver sucks? No...it just means it needs improvement.