Part of the beauty of Bulbasaur is that its Japanese name, Fushigidane, can be taken to mean "mysterious bulb" OR "isn't it strange?" This is relevant because Bulbasaur is the only dual-typed unevolved starter, making it strange. If Fennekin were to be dual-typed, the pun would be lost.
I do not support Fennekin being Fire/Fighting. Its evolutions, on the other hand.
-One hundred and nine characters of space for rent-