The problem is that making those available to the public involves cutting an access route through the armor. He said most battleship museums do this through turret two because then you only have to cut one hole to access both the upper and lower turret. Since we have to do two, that doubles the time and money it will take to get the turrets open.
Having been aboard the Showboat a few years back, I can state that they didn't have access to the gunhouse through the barbette. Turret #3 had the only open gunhouse when I was there (apparently, #2 has since been opened up, but this was 2010), and access to it was through the normal entrance hatch on the bottom of the turret overhang into the turret officer's booth; the gun rooms were closed off, but with plexiglass covering holes cut in the hatches to them to allow you to see in.
They
had, however, opened up barbette #2 with... either one or two hatches cut through the armor. (I think two, actually--one for entrance, one for exit.) You entered through the upper level of the powder handling room, IIRC, then took existing hatches and ladders down to the lower level and through an existing hatch to see some of the magazine, before returning up through existing hatches and ladders to exit through the lower shell room. Since the only access to the turret stalk while in operation was via manholes in the floor of the handling room, the hatches and ladders to get to the other levels should already be present.
(I must say, the hatch through the barbette is most impressive in its thickness... and in it being such a perfectly clean cut that I initially thought it was an original hatch used for crew access originally, before I learned more about that!)
And I was under the impression that on the Iowa's, at least Missouri and Wisconsin, the only Class A armor was on the turret faces, the rest of the heavy armor was Class B... dunno if that's the same for Iowa and Jersey. Have to go dig up my Friedman to make sure, it's been a few years since I read it.
I think the barbettes and conning tower were also Class A, because of the fact that, unlike much of the belt, what was behind that armor was packed as densely with Important Vulnerable Things as a tank, and because the shape meant that they couldn't effectively incline it or rely on angling the ship to get an oblique impact. (Yes, I know, cruisers had conical barbettes to incline the armor. That was an expedient BuShips never really liked, as evidenced by the Des Moineseseseses going back to cylindrical barbettes, once the treaty limits were lifted.)
