Smith, who signed a futures deal with the Lions following the 2012 season, joined their practice squad last October. He originally entered the NFL by signing with the Dallas Cowboys as an undrafted free agent out of Texas A&M, and he ended up being placed on the Lions' practice squad injured reserve shortly after coming to Detroit.
With Smith being released, the Lions have nine cornerbacks on their roster right now: Chris Houston, Darius Slay, Jonte Green, Bill Bentley, Ron Bartell, Chris Greenwood, Ross Weaver, D.J. Johnson and DeQuan Menzie. Guys like Houston, Bartell, Slay, Green and Bentley seem like locks to make the 53-man roster, and Greenwood seems like a pretty good bet to make the team as well.
In other words, Smith's chances of making the 53-man roster were slim, so he was deemed expendable with the Lions needing to open up a spot for Harris.