Most people want Tierney because he's showing all the signs that he's going to be one of the best left backs in Europe, so signing him will mean we have a World Class left back in a couple of seasons that we can either keep and kick on with or sell for a huge fee
As Digne is older, the chances of him kicking on and making significant improvements now are less likely
Also, we can ease Tierney into the first team over the season and gradually ease Baines out, which will be a smoother process then essentially kicking Baines to the curb as soon as Digne signs
Tierney is just the better option, which isn't to say Digne is bad of course, but the point stands