This is a limitation of Apple's Thunderbolt display, not Thunderbolt itself. Thunderbolt is designed to allow displayport at the end of a chain. For some reaso

This is a limitation of Apple's Thunderbolt display, not Thunderbolt itself. Thunderbolt is designed to allow displayport at the end of a chain. For some reason, Apple chose not to support connecting a displayport adapter directly to a Thunderbolt display, but you can put a daisy-chained device in-between your Thunderbolt display and your displayport display, and it will work.