From my understanding of Tau fluff the weekends in hand to hand stems from several factor.
First is cultural, hand to hand is seen as primitive and savage unless it is ritualised (ethereal and there honour blades). Also as has been mentioned Tau are more like human today in terms of killing, eg. At range no problem, but close up it become more difficult for sociological reasons.
Second, early description of Tau eyesight always made me think of them as being chronically farsighted compared to average humans, so they can see great amount of detail in the distance, but thing with in a meter or so a slightly blurred compared to average humans, so when in hand to hand, where the struggle is quick it is harder for them to react successfully.
Thirdly, guardsmen are trained both in hand to hand and range, hence WS and BS of three, while Tau are only trained at range. With minimal hand to hand, only the basics as a necessary evil in case it comes to that. conscripts, which are portrait as humans that have not been really trained but just given a gun are about as good as Tau in hand to hand.
So between training, there eyesight and possibly other minor things like that and cultural issues it adds up. Also consider Tau trained under Farsight, they are as good as Gaurdsmen in hand to hand skill wise. So training certainly plays a role.