The thing is that Thunder is very reliant on rain to work, which HO teams often don't have the slot to use. Of 'course if you are facing Tentacruel its probably going to be on a rain team with rain already up, but using an unreliable move that hits reliably only one Pokemon (and generally hits only one Pokemon that Gengar really cares about) is probably worth more of AC that main slash. This said, however, this doesn't mean that Thunder + HP Ice won't get a big mention in AC, as i have used it and it is indeed very useful against rain teams.