The problem with the bat is that it is much slower, it is almost entirely useless at close range and you have to commit a lot more to swinging a bat. A bat swing leaves you much more vulnerable than a knife would.
At the beginning the bat has the edge, but the first swing is going to determine the fight. (Assuming both people know what they are doing). If you are going against someone skilled with a knife, you really only have 1 shot. If the first swing misses you are stabbed.