halacha theory - Why don't we just follow the shittah of Moshe Rabeinu?

My question can apply to virtually all mitzvos where there is a disagreement in the standard way to fulfill them.

If one reads the hakdama (introduction) to the Rambam's peirush mishnayos (commentary on the mishna) he explains the origin of machlokes (disagreement in halacha).

But if Moshe was the main Rebbe, why didn't everyone just follow Moshe?

For example, knowing what Teffilin Moshe wore should resolve the disagreement over what our Teffilin should be like. Why didn't everyone just follow his shittah and pass that tradition down to us?

Edit:I found in the Ben Ish Chai Shana Alef parshas vayeira 21 that from Moshe Rabbeinu until the Geonim people wore two pairs(Rashi and Rabbeinu Tam)