The hebrew word for song, Shir, literally means a circle. A circle denotes completeness, for you cannot add to its length without breaking it, and it has no end. Moshe Rabbeinu summarized history in a song because history cannot be modified substantially, only details can be filled in, and it will ultimately reach its completion no matter what those details are.
Like a song, Moshe Rabbeinu's life could only be precisely 120 years. His life was a saga of completeness; any addition would have detracted from its perfection. He was also the one who transmitted to us the written Torah that, like a song, can only be written in a prescribed manner with specific letters. Not one letter may be added or subtracted. Thus is the nature of perfection - it must be all encompassing within the confines within which it is perfect.
There is perfection yet beyond perfection: limitlessness. Perfection is limited to a confined spectrum, whereas limitnessness is limitless. Yehoshua's leadership had a tremendous advantage over that of his teacher: it wasn't perfect. He, and all the leaders of the Jewish people after him who similarly transmitted the oral tradition, worked towards perfection. Slowly, they are bringing us closer and closer, yet until utter and complete perfection will eventually be reached there is no limit to how much you can do. Such is the nature of the oral law that Yehosha represented, it can be expanded and expanded until it encompasses all the endless meanings ensconced in the limited number of letters written on the holy Torah scrolls.
Hashem is called Ein Sof, endlessness, and the more energy we spend delving into His endless Torah, the closer we get to him.
Adapted from Rabbi Vali's book on Dvarim. Questions and subscriptions can be mailed to: the Yeshiva