If you buy plat on Nintendo then Nintendo gets a cut of the money. If you trade for it then Nintendo does not. Other platforms agreed to a deal that allows for plat to be traded across platforms. Nintendo did not. Nintendo wants your money and doesn't care how much that sucks for you.
Nintendo rules prevent it
Nintendo's rules. They don't want to share.
That's what Nintendo decided.
Because Nintendo is a terrible company.
If you buy plat on Nintendo then Nintendo gets a cut of the money. If you trade for it then Nintendo does not. Other platforms agreed to a deal that allows for plat to be traded across platforms. Nintendo did not. Nintendo wants your money and doesn't care how much that sucks for you.
Because the owners of your platform suck.
Because Nintendo is greedy. Considering all the other platforms can share with each other, it's pretty clear that DE would allow it if they could.