It was supported by stating that a reliable fabric supplier has been found and that a major competitor is no longer making alpaca overcoats, because of which demand will be high.
Why is there a comma in this sentence?
It was supported by stating that a reliable fabric supplier has been found and that a major competitor is no longer making alpaca overcoats, because of which demand will be high.
Why is there a comma in this sentence?
"Because of which demand will be high" is a relative clause whose antecedent is most likely the phrase "a major competitor is no longer making alpaca overcoats." Relative clauses whose antecedents are themselves entire clauses characteristically use the relative pronoun "which" and are surrounded by commas.
"Because" here is not introducing a subordinate clause on its own; instead, it's part of the prepositional phrase "because of which," which contains a pied-piped relative pronoun.
Generally speaking, a comma before "because" tends to be dealer's choice, more an issue of style than grammar and the choice to put one becoming more likely when it eliminates possible ambiguity.
At first, I considered that it's not what it appears, not simply a subordinate clause introduced by "because" but a nonrestrictive relative clause with an inverted structure that doesn't put "which" first, which would nonetheless require a comma, but it isn't. You can tell it isn't because you can readily substitute "that" for "which" and have it mean the exact same thing. Since "that" only introduces restrictive relative clauses and does not require a comma before the clause and since an alternate meaning of "which" is "that," even if the relative pronoun were the operative word and made that a relative clause, it still wouldn't require a comma, making any statement of "should" not on solid ground, but I ultimately decided that's not what it is. Just know that it wouldn't have mattered even if I had decided that's what it is or if that were what it is.
So why is it there? Because the writer chose to put it there, maybe feeling the sentence reads easier with it there than without it.
But "should" it be there? It "should" if the writer is obliged to follow a style guide that requires it. Otherwise, it's a matter of opinion since it's a matter of style, not grammar, making "should" an awfully strong word, probably too strong a word.
I will say that I like it there. I do think it reads easier with it than without it.