The Rambam in ספר המצוות, addresses the question as to why מרור is not considered a Torah law but rabbinic. The question comes about because the Torah specifically says regarding the קרבן פסח, that it shall be eaten על מצות ומרורים יאכלוהו. This would seem to indicate a Torah commandment to eat the bitter herbs.
The Rambam explains that the מרור is really a supplement to the primary Mitzva of offering and eating the קרבן פסח. This is similar to other sacrifices where there is a primary commandment to offer a particular sacrifice. This is followed by the details connected with it. For example, frankincense and wine are included as items to supplement the specific sacrifice. There is no separate Mitzva regarding frankincense or the wine libation. Like the מרור they are details regarding the Mitzva, but not a Mitzva in themselves. Hence, the מרור is a rabbinic law at our Seder.