I didn't find anything in classical sources - so here is my suggestion.

We are told that the Torah is an "Etz Chaim" (Mishlei 3:18) - a life-giving tree, which continues to grow without measure (Iyov 11:9, Eruvin 21a).

Only G-d-granted wisdom can grow in that manner; man-made wisdom cannot imitate that trait. Therefore we are prohibited from planting a tree, or even putting a wooden beam, in the Azarah - the place of the Shechinah's presence (Devarim 16:21, Tamid 28b). Our "tree" is infertile and cannot grow, compared to that of the Written Law. This was the metaphor of Aharon's sprouting stick - he correctly understood how to interpret Hash-m's Word with His 13 Midos (Bamidbar 17:23); "The lips of the Kohen keep Divine knowledge" (Malachi 2"7).

The laws of Chol ha'Mo'ed are mid'Rabanan, or at best Unwritten Law (Chagigah 18a) which is interpreted by the Rabanan. Unlike the Written Law, they are "barren", and one is not supposed to "build" on them with his own logic. A similar statement is made regarding the laws of Tereifah, which are also Unwritten (Chulin 48b).

Best regards,

Mordecai Kornfeld