Why not succession by seniority (next oldest member of the family) instead of primogeniture?

Sucession by seniority has some advantages – it’s another simple metric for sucession. 

However, it has some disadvantages – it tends to lead to gerentocracies, it complicates marriage alliances, but most of all it leads to conflict between rival branches as now the brothers and cousins and uncles are now vying for power as well as the sons.