Because using a semicolon with a conjunction is generally wrong. One should use a comma.
PS - I usually drop the required comma before conjunctions if the sentence seems to flow fine without it. I'm not recommending that, just explaining my posts
I figure most people don't remember that rule!
I suppose that someone who ignores that rule that requires a comma between independent clauses might think that a semicolon would be right in this situation. Neglecing the comma makes it look like the clause is dependent, and that usually changes the meaning. The comma does make a pause, and that's what is required. The rule isn't for appearance; it is for meaning.
I was explaining my forum posts, because they are different than my formal writing, or writing to be published. I always try to note that here, because this forum is about the correct way to do it
I'm still not convinced that 'then' is the same as 'but' or 'and,' btw
I'm going with some of the others on this one, respectfully