I think there are three main reasons for lack of wealth.
The first would be the fact that the heir usually received the lot. Younger male siblings were largely forgotten, and females were expected to marry off into money. The little money they did receive would see them confortable in their own lifetime if they were lucky.
The second would be marrying downwardly. I descend from a legitimised daughter of James IV (Scotland). As the daughter of the King, she married a Earl. The children of that marriage bar the heir married into other noble families, some with lesser titles. In my case we end up with the Abernethy family who only 200 years after their Royal link were marrying title-less Lairds. In my case, the Fraser of Philorth. The Frasers usually married in to local families. Through this process, the people become less wealthy, and less influential.
The third reason is illegitimacy. In the case of my greatx6 grandmother, her lowranking Noble (if that father) left her only £10 in the will. He had no other children, but still left his remaining thousands to his nephew, the Fraser Lord Saltoun.
So in short, if you are not the eldest son, of the eldest son, of the eldest son, there probably wasn't much money passed down anyway. What was left became less and less over time. And if you are illegitimate, you shouldn't expect anything.
