This is an issue of safety. When an acid mixes with water, it will react very quickly. This reaction can release a large amount of heat and can cause the solution to bubble or splash. It’s important to pour an acid into water, since acidic solutions are almost always more dense than pure water, meaning that the acidic solution will sink down into the water as it reacts. If you were to add water to a more dense acidic solution, the water would react at the surface of the solution, and there is a much greater chance it could bubble or splash up at you.