There are, apparently, too few modelers who want to model ships from the Viet Nam Era. Or, perhaps there are still too many modelers who want to model WW II.
Kits are produced, depending on "the market", so, as stated above, the " market " is WW II, and not the Viet Nam Era.
With my economic status such as it is, my options are: Scratch build; Kitbash; or take up knitting. I won't knit, I can't afford the "bigbucks" kits, so I guess my options are reduced to scratch build. Kit manufacturers, I'm not the only modeler outside your "target audience", but keep up the good work, because as long as you do, the models I build will remain unique, and the kits you produce will remain " un-purchased by me ".