What is the reason that the right strap is longer than the left strap by the Tefilin Shel Rosh?
The Shiurim for the length of Tefillin straps is one of the many halachot of Tefillin that is Halacha LeMoshe Misinai. There is no reason given for this in the Halachic works.
That being said, Kabbalah and Chassidut bring many explanations for this. The general idea being that the Tefillin Shel Rosh has four compartments for Chochma, Binah, and two types of Daat. The two parts of the strap that surround the head represent Chesed and Gevurah. The kesher is Tiferet and the two parts that descend in front of the person are Netzach and Hod. The right one being Netzach and thus longer. The longer strap should descend to the place of the brit which is Yesod.
If you would like to understand this matter in depth, I would highly recommend studying the Drush about Tefillin in Derech Mitzvotecha of the Tzemach Tzedek. Here is a link:
It is repeated in the name of Rabbi Zamir Cohen.
Since the right side in Kabala is Chesed and the left side Gevura - in order to have more Chesed than Din we make the right strap a bit longer.