All who followed the Naruto/Boruto franchise know byakugan as one of the most protected kekkei genkai in Konohagakure!, The Hyuga clan doesn't play around when it comes to that, they even go as far as marrying only from their clan, just to keep Byakugan tightly locked in their clan, here's why they do this!
First and foremost there are two branches in the Hyuga clan, the upper branch (the main branch ) and the lower branch, The lower branch exits to serve the upper branch, many say this was done to protect the secrets of the Byakugan from outsiders, but it goes Dipper than that.
Hiashi Hyuga always wanted to protect the secret of the byakugan because it was indeed an all-powerful Dojustu in the hands of any who's well equipped, Secondly, he wanted to keep the upper branch kekkei genkai bloodline pure, so that no flaw would hinder the upper branch from attaining a pure Byakugan, However, this tradition has been going on for ages, as far back as when Humaru Ōtsutsuki left his bloodline on earth (they were later known as the Hyuga clan) to the moon.
Later thanks to Toneri Ōtsutsuki we all were informed that with a pure Byakugan one can Attain Tenseigan, Byakugan itself is already an all-powerful Dojustu, what makes Tenseigan Special, well Tenseigan allows its user to control both attractive and repulsive forces, similar to the Rinnegan. it also grants its wielder Tenseigan Chakra Mode, which in turn gives the user an increase in speed, power, strength, durability, and reflexes, in order words Tenseigan is an overkill Dojustu, for one to make use of Tenseigan he/she has to have Alot of Chakra reserves, due to this limitation Many Hyuga as of then, do highly skilled they never once activated Tenseigan.
Byakugan is indeed a powerful Dojustu, The same can be said about his later Tenseigan, With how powerful the Hyuga kekkei Genkai Seems to be, there was a need for restriction in place, for it not to fall into the wrong hands. The Rule/Law to protect the Hyuga kekkei Genkai ( Byakugan ) was a bit out of place, many understood the greater good in keeping such weapon of mass destruction in check and thus accepted such a compromise.
