Just like sharingan has to be awakened, byakugan can also awakened through some special event.
Because the fact that we all know that sharingan is the gen that coming from Kaguya, who has sharinnegan. But after giving birth to Hagonomo and Haruma, her son has only rinnegan and the red rinnegan without tomoe / her other son only has byakugan. This can be explained that her gen is being divided through mating with another person (without sharinnegan ofc) and giving birth to hybrid.
Same to indra and ashura, we only see indra version with sharingan, so i can assume that his eyes is always active since birth (like a normal eyes or like how byakugan works).
With all the given fact, after indra mating with some unknown, until the day the clan Uchiha is formed - Their eyes need to go through some special event to awaken the special gen in their eyes/brain to activate the sharingan.