"Since this matter is not about you, don't worry."

Translation: Ĉar tiu afero ne estas pri vi , ne estu maltrankvila.

[Kreilyn]

Why not ekde?

[miacomet]

Ekde indicates the beginning of an action (it's formed from ek- which means beginning or start, and de which is "from") whereas ĉar indicates a reason. If you can substitute "because" then use ĉar and if it's about time, use ekde.