Why can supplement products not be subjected to testing against "everything" on the WADA prohibited list?

The list is not definitive ie. it is general, and potentially infinite in length (see here).

The list is specifically designed for urine analysis and refers to substances found in urine ie. after the drug has passed through the body. The body may convert some or all of the drugs to other compounds as it passes through the body. Not all of the substances on the WADA prohibited list are able to be present in a supplement product.