Recently I've been trying to get people who frequent Reddit into watching / reading Guyver and most have never heard of it. Now the 2005 series might have helped its popularity a little or the fact that there are still models and figures floating around, but it really hurts a manga's popularity when you can't find it on the shelf somewhere. Both Guyver and Berserk came out around the same time, and yet Berserk is actually still popular, dare I say mainstream. Every time you go to Barnes & Noble or any other bookstore, there it is on the shelf. Everybody I talk to knows what Berserk is, but nobody knows what Guyver is.

So I'm thinking part of the problem with Guyver being unknown in the US is that Viz stopped localizing it(I won't blame them if it was out of their hands though). Did they stop because Takaya was already slowing down the releases? Or was there a big decline in Guyver's popularity back then? Was it not selling well?

 

A combination of all you just pointed out, and I think they were having copyright issues as well for Guyver (?).  It's always been sort of a tiny niche manga.