That's because the species of banana they flavored candy loosely off of the gros michael died off due to not being resistant to panama disease. Thus the Cavendish banana became the standard because it's resilient. You can find heirloom bananas that taste pretty close ( useless knowledge I have a crap ton of). It's not 100 percent the reason but it's basically the chemical that creates banana flavor is much higher in the old variety so it resembles it more closely. Still not a 100 percent match but it is closer and understandable why the flavor is so off from bananas now.Ever notice how they don't taste anything like a banana though?
It's kind of like the flavor purple.