Moadim Lesimcha. One of the Halachot related to Succa, is that a Succa that is higher than twenty Amot ( about thirty-five feet ) is פסולה, not valid.
There are two reasons given for this Halacha. The first is that twenty Amot is considered more of a permanent dwelling. The Succa was meant to be a דירת ארעי, a temporary dwelling.
The second reason is that the Torah says, למען ידעו דורותיכם, in order that the future generations know that Hashem prepared Succot for us in the desert. A twenty Amot Succa is not adequately perceived by the eye.
The ultimate purpose in either case is to recognize Hashem’s abundant kindness and our complete dependence upon Him. This is the message of Succot.