Why was the daughter murdered in murder on the orient express?

One assumes you are talking about the unfortunate Daisy Armstrong, daughter of Colonel Armstrong. If this is the case, there is no official explanation but the assumption is Ratchett/Cassetti (the murderer) killed Daisy because he could not be bothered to take care of the young girl. After all, he assumed he'd get the ransom money anyways. 

Communicating with the dead was common, it wasn’t seen as bizarre or weird,” explains murch. “it’s hard to imagine that now, we look at that and think, ‘why are you opening the gates of hell? ’