- What your asking is actually been explained it is a Rare ability. In the same light all the members of the Senju clan have above average chakra reserves and powerful lifeforce energy but Hashirama is the only one to every gain Mokutan abilities. Just having powerful life force energy is not enough its more than that.
- In the same light perhaps not all Uchiha awaken their Sharingan and only a handful can awaken their MS. Not all Uchiha eyes are the same some have more potential than others. The same can be applied to the Byajugaun as we know some members have superior vision compared to others.
- Differences can exist even among people who belong to the same clan. Obtaining the Rinnegan is not as easy as just throwing some random Uchiha & Senju member together. They too must each have outstanding genetic ability to increase the odds of obtaining the Rinnegan.
Also don't forget the 2 descending clans where rivals for thousands of years hence why they did not mix blood with one another. The Uchiha clan did not mix with the Senju for obvious reasons they are trying to maintain their Blood limit which means intermarrying within their own clan. Sasuke's father & mother where both Uchiha. Their eyes are clans power and maintaining that is of the highest order !!!
The Same can be true with any clan with unique genetic abilities like the Hyuuga etc.
You don't think that someone from the Uchiha clan can just go and marry anyone they please without any consideration of their genetic legacy. It parallels reality in some ways in that the son of very rich successful parents would be pushed to marry someone who is also comes from a similar outstanding background rather than some girl from a poor neighborhood.