Well there are a couple of good reasons.

Pine trees grow both straighter and taller than Birch, Beech, or Oak with fewer flaws like knots and the weak points in a structure are where beams are joined. So a taller straighter beam allows for a sturdier structure. (This is why White pine trees were used for masts on sailing ships and why President George Washington set aside several acres of White pine forests as a strategic reserve).

Hard woods like Oak, Birch, and Beech are harder to work with especially when you haven't got access to iron. The Beech in question is likely the European Beech tree that yields a tough but dimensionally unstable beam.

I'm not sure why the developers decided that Beech trees were a soft wood equivalent to a fir tree and fir and pine logs are not suitable for fires because of the smokey residue left behind.