I think it's because you really have to search out Thunder Force in the States where as Gradius and R-Type are much easier to come across.

Doesn't help that we haven't had all the Thunder Force games released here and what we have has a re-title or two in the mix.

So Thunder Force...highly underrated. 3-5 are awesome and, yeah, I'd say better than recent Gradius and R-Type instalments. Good stuff. I'm not a huge horizontal shmup fan, but I always dig Tecnosoft's series. (Compile too. )