If I was to guess, in all fairness I can see potential logic behind TW keeping the Banner Editor basic for now. In a highly customizable editor, especially if image importing is allowed, it's inevitable that some players would make uh... "controversial" flags and post them online, that could potentially look bad on TW's behalf for "allowing this to happen". We already seen this happen in early EA with the online banner editor and in other games.
Of course this hurts the vast majority of players who don't want to act like edgy trolls, but I can see why it would be a valid concern for the devs. They can absolutely expand the current editor to give more freedom, but I can understand why they might only do it in a controlled/limited way.