How come that Mount Sinai is not regarded as an important sacred site as the Kotel for instance, after all it is spoken of many times in the Bible?

In order to indicate that the Torah is not bound by time or place. The poles used to carry the Holy Ark were never dismantled, in order to indicate that the Torah is always available to go to any situation. The Torah explicitely says (Shemot 19;13 that at the conclusion of the giving of the Torah at Sinai the restrictrions of going up the mountain will be removed.