Though in the definition listed above since appears to relate only to time, in reality, people use it the same as because to imply cause.
Notice the similarities in these two sentences:
Since he ate cookies, Charlie is a happy boy.
Because he ate cookies, Charlie is a happy boy.
In the sentences, both since and because are helping us show the cause of Charlie’s happiness: eating cookies.
However, using since and because interchangeably can cause problems when it is unclear whether since is referring to time or to cause.
Notice this sentence:
Since he slayed the dragon, Charlie got measles.
In this sentence, it is unclear whether Charlie got measles as a direct result of (caused by) killing the dragon or if he simply contracted measles in the period of time after killing the dragon (not caused by the killing).
If we used because in the sentence above, we would know the dragon slaying directly caused his measles. As it is written with since, the causation is unclear.
To avoid confusion, it is best to limit since to time elements and not use it interchangeably with because. Because is the best choice to indicate directly the reason something happened.