The problem is not necessarily that these expanded tastes do not live up to the original; indeed, as is generally the case, established candy brands become successful because their original incantations are so good. It is only natural that their sequels will pale by comparison. The Godfather IIs, Empire Strikes Backs and Wrath of Khans are the exception rather than the rule. (In the candy realm, examples of candies that have improved upon the original: the peanut M&M, the addition of the fruit punch Sweetart, and the gummi worm--worms are somehow better than bears.) Yes, the originals are almost always the pinnacle.
However, the tropical flavored candies do not even approach the appeal of the original brand products. Let's take Starburst Tropicals as an example. Was there a group of people out there clamoring for "Mango Melon," "Pina Colada," or "Strawberry Banana" chews? Or, in Skittles, was there a call for "Banana Berry," "Kiwi Lime," or "Pineapple Passion Fruit?" Or, in Mike and Ikes, a grass roots movement for the similarly-flavored "Kiwi Lime," "Strawberry Banana," and "Pineapple Banana?" I would think an answer of "no" to all of these questions is logical. However, all of these brands are pushing these inferior products into the market.
There are a number of reasons that I object to these creations. First, only the vaguest notion of "tropicality" ties the flavors together. Second, the American market is most likely not able to identify some of these flavors-- do we eat a whole lot of passion fruit or kiwi on a daily basis, to the point that we want a candy of those flavors? Third, because these flavors are largely foreign to most, candy producers have no real incentive to get the flavors right; and, since the formulations for these new candy flavors are not widely developed (as are orange or cherry, for instance), even those corporations trying to get them right often fail. Fourth, it seems that corporations do not trust the ability of any "tropical" flavor to stand on its own. Perhaps owing to their inability to develop "kiwi" or "passion fruit" just right, or rightly understanding that no one really wants those flavors in the first place, candy brands force together nasty duos of flavors - one flavor we know and one we don't know or like as well. Every tropical flavor is some haphazard hybridization of often incompatible tastes. Why spoil strawberry with the worst candy flavor possible- banana? Why force kiwi upon lime? Why mask pineapple, a flavor that Americans might know and like, with passion fruit?
In the end, tropical flavors are an unfortunate result of the move toward brand expansion. Sure, the gummi Starburst is a solid product, and the Sour Skittle is a triumph; all expansion is not bad expansion. However, some arenas should not be populated, and tropical is one of them. Perhaps candy companies should have tried to expand into that market, but the fact that they persist is unconscionable.