Sukkoth is called Hagh Hasukkoth (Festival of the Booths) because of the commandment to dwell in booths. Pesah is called Hagh Hammasoth (Festival of the unleavened bread) after the commandment of the holiday.
Shabu'oth (the Feast of Weeks), however, is named after the counting of the days before the holiday which is simply a preparation for the festival itself. Would it not have been more appropriate to call it Hagh Hattorah (Festival of the Torah), because this is the day in which the Torah was given and is the essence of the holiday?
In Ben Ish Hayil there are several explanations. One of them is that the seven weeks contain 49 days. The letters of the names of the 12 tribes of Israel also total 49 (49 letters when Binyamin is written with one Yod).This comes to show us that the Torah belongs to the sons of Jacob (the twelve tribes).
(See Ben Ish Hayil, Derush Beth LeShabbath Kallah)
