Frogs are amphibians, and therefore cold-blooded. Kissing a prince should be a warm, mushy experience. I expect the frog's room-temperature body would be as corpse-like in feel as anything I can imagine. Who wants to buss a stiff?
They are also small and green with spots and bumps, and slimy. Not exactly the qualifications one would look for on a prince resume. Physiological oddities may have their attractions, but I would not call them charming.
One expects a prince to have numerous talents. After all, they spend their upbringing practicing sword fighting, jousting, playing instruments, and dancing. I can't imagine a more unbard-like song than the humble ribbit which the frog throat emits. Croaking is just not romantic.
Besides all that, they have rather fat bodies with ridiculously long hind legs and enormous webbed toes. As a dancing partner, they would not be easy to work with. Out of charity, one must make allowances, but there are some adjustments that are just beyond virtuous extravagance. Frogs' legs may have their delights--on the appetizer tray rather than the dance floor.
Finally, their wide-mouthed grin and long-tongued attraction to flies and other winged insects is a real put-off. Even if you could find it in your heart to feel friendly to a green guy, it is so disconcerting to be in the middle of an intimate conversation, only to be interrupted by a "thwip", as that impossible lasso zips out of his toothless mouth, followed by a decisive snap as his jaws clamp down on the hapless arthropod. Invariably the wings will stick out on each side, until that enormous "glug", when he manages to slide the disgusting snack down his throbbing yellow throat. He then reverts back to the wide-mouth grin without anything close to an apology. He can't help it! And there are so many insects out on full-mooned nights which invite the kind of intimacy to which I was alluding.
There are just too many allowances to be made for frogs to qualify them as bona fide royalty. Princedom can only bend so far.