My understanding is that yoghurt is is the biproduct of a yoghurt culture, a bacteria, eating lactose and excreting the yoghurt. 

My question is - why does it need to be lactose that the yoghurt culture is eating? 

For yeast to produce alcohol, for example, the yeast can eat any sugar, and it's up to the brewer to decide whether that's sucrose, corn, barley, grapes etc to affect the taste. 

Why is that that yoghurt cultures need lactose specifically?