I think it is because the chocolate coating is very thin on Bounty bars, so the taste isn't overly strong. Also maybe dark chocolate works on Bounty whereas it may not do so well on other bars.
I am still surprised they haven't done a white chocolate Bounty, although I suspect it may be a little too sickly. I know you hate white choc oldspice, but there are deffinitely some bars that taste better with it - white chocolate Twix was far nicer than the regular version.
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