In raw grains, minerals are locked away behind a chemical defence mechanism designed to prevent digestion. This lock is called Phytic Acid (or phytate). Unless you have the key to open it, you are likely excreting the vast majority of the nutrition you think you are eating.

When you feed your starter exclusively with white bread flour, you are effectively putting your microbial colony on a diet of pure sugar while denying them the vitamins they need to metabolise it. You are creating a starter that is energy-rich but nutrient-poor.