Because two years ago with the Browns he was regarded as fast on his way to becoming one of the best corners in the league.
His yards per play and success percentages, as measured by Football Outsiders, were absolutely stellar. He tackles, comes up in run support, can intercept the ball and play man coverage. He went to that Tampa Cover Two up in Detroit and he flopped around. It didn't work out for him. The entire defense was terrible and the coaching was terrible.
The reason he's being released is they traded for him and gave him a contract that was basically a one-year tryout after which they would decide whether to pay him an $8.6 million roster bonus, or cut him. They're choosing to cut him. Why? My guess is because the Lions want to focus their resources, for now, on the classic method of rebuilding a franchise from bottom-up. The same way Miami did it. By focusing on the Offensive and Defensive Lines. They're not about to commit $9 million of precious cap dollars to the CB position before they start developing the lines.