For karpas at the seder, one is supposed to eat a food on which the blessing "borei pri ha'adama" is made. However, many of the most commonly used food items for karpas (e.g. parsley, raw onion, raw potato) arguably might not fall into this category. Wouldn't we all be better off using bananas?
It's supposed to be something that would be used as an appetizer at a fancy dinner or the like. So would a banana fall into that category? If anything, I would think that would be more of a dessert. (Also, would you want to dip it into salt water?)
Granted, onion and potato aren't really used as appetizers either, but back in the "old country" those were often the only vegetables available, so some people still use them because of tradition.
In your question, you alleged that "raw potato" would not fall under this category. I would challenge your assumption that anyone would eat raw potato; usually a boiled potato is used (which looks quite similar to a raw one but tastes a lot better!)
R' Teitz, from Elizabeth NJ, was known to use bananas for this reason, as well as k'dei shehatinikos yesha'alu [to get the kids to ask] as well as to remind people that their bracha was ha'adamah, and not ha'etz.
The problem with any karpas that's too tasty (including boiled potatoes) is that by the time people get to the seder they're hungry, and I was always told you were supposed to eat just a little bit of the karpas, less than an olive-size piece ("Ke'zayit"), to avoid the need for a after-bracha. If we served bananas, people would gobble them down.
Bananas are a three way argument what the bracha is. Two hold it is Hoadama, but for different reasons and one holds it is Haeitz. So you are better off with something whose bracha is for sure Hoadama.
What I mean is that although the final psak halacha is that bracha for bananas (as well as pineapples) is hoadamah, nonetheless since it is an hoeitz according to one of the three opinions brought by the poskim (halachic authorities) and since the halacha of karpas is to specifically use something whose bracha is the same as the bracha required for maror (which if you are using romaine lettuce is hoadamah, but if you are using only horseradish then it is a question because it is more likely a shehakol since it is not normal to eat it raw on its own) better to use something that all agree is a Hoadamah.