New wine must be put into new leather bags. The container is changeable, the content isn't. In other words, the Church needs to change it's delivery system to match the needs of those they're delivering to. It never changes its message, but it adapts how it delivers that message. Otherwise, as Jesus says here, the wine doesn't get to its intended destination.
New wine explodes old bags.
And everyone loses. To the Pharisees, fasting has become a sacred method of achieving spirituality and acceptance with God. Fasting's good for some occasions and not for others. In other words, He's illustrating the truth that you can change your style and method as needs change. The relationship remains constant.