Because the priest uses his FP to infuse the water with Holy +1. But the amount of FP is fixed, based on the current level the priest has. This means the more water the priest tries to infuse, the weaker the effect. When infusing a small vial of water, it's very effective. For a vampire it would be like throwing scalding hot water at a regular person, burning them badly. But if you try to do something larger, it doesn't work as well. Once you get to the size of a coke bottle, it would be like just luke-warm water, not doing a lot. At that point, you'd be better off throwing pocket sand.