Why has my crape myrtle lost it's pink color?

As far as I know it, the pH of the soil may well be the culprit. Just like the hydrangea bushes in my yard need an occasional boost with:Miracle-Gro Nursery Select Miracid Water Soluble Plant Food - Scotts

A special plant food that creates the ideal growing environment for acid-loving plants like azaleas, rhododendrons, camellias, dogwoods, magnolias, gardenias, orchids and all evergreens Rich in iron and other essential nutrients.

I think that you would do well to seek out a plant food that will increase the alkalinity of the tree, as well as the soil encircling it.