In the proofs that I submitted with my assignments, I used the logical symbol for because (the therefore symbol upside down) and the marker crossed it out and replaced it with "since". Why?
The symbols and are much less known than, e.g., . My opinion is that unless you know that the person reading your proof is aware of them and does not mind them, you should not use them. Or, at least, you should make a note explaining what these symbols mean.
The purpose of a proof is to communicate an idea not to raise barriers to communication.
"Or the person reading could look them up..." or they could decide that we are so arrogant we are not worth reading.
The vast majority of students passing through maths courses are not destined to be mathematicians, in which case they should be taught in a manner that does not raise barriers against understanding. You are not going to make them mathematicians by making them jump through inappropriate notational hoops.
I always thought symbols were much more clear than words (e.g. logical "exclusive or" symbol versus writing the word "or"). I was not penalized, so it is perhaps a stylistic preference of the marker as the word "since" exactly means "because" when it is used as a conjunction.