There are a lot of people living in Bangladesh. It has one of the highest population densities in the world. There are 165 million people living within 57,000 square miles of each other. Despite that large population, most of the population has access to safe water. Only 5 million people lack safe water access; however, 85 million people lack proper sanitation.
This question can be answered with standard ecology reasoning. Bangladesh, like any other population within an ecosystem or habitat, has a finite amount of resources to support any given population. These resources are often called limiting factors. Limiting factors abound, but in general, they can be simplified to provide a more concrete example. Let's work with three limiting factors. Those factors will be food, water, and shelter. If a population (of people, animals, plants, etc.) has access to unlimited amounts of all three, then the population can grow without consequence. Unfortunately, those factors are not unlimited. Even with two of the three being unlimited, the third factor will limit population growth.
For this question, Bangladesh's population has overshot the water limiting factor. The amount of water present is not enough for the current population; therefore, the population has outgrown the area's carrying capacity. As the population grows, the problem gets worse. If there is only enough water for 160 million people, adding more people will simply increase the number of people that do not have access to safe water. A growing demand for safe water will occur, but it is somewhat irrelevant. Demanding and getting more water only works if there is more water to be had.