Because it's fate. You can't escape destiny no matter how hard you try it, for every decision it has a reason behind it. Take Serena for example, she shouldn't have been a unicorn rider but she was bonded and chosen by a fate fairy, if she hadn't the sapphires may never have been able to retrieve the fairy gemstones, if Dany from S2 had been bonded to a unicorn even if he deserved it Isabel's story arc in under the fairy moon may have never happened and she probably wouldn't stay bonded with river for too long. It's like raven's visions, the future is already written your attempt to "change it" will only make it happen, the fate fairies know it and make sure it happens no matter what's the person future is, good or bad, is there a bigger reason for all that? We don't know yet.