Celle is used because, as a pronoun, it directly replaces "robe" and works with the relative clause to specify the dress, while laquelle would imply a choice or question about a group of dresses.

Choisis laquelle tu veux. -- Choose which one you want. (from a selection of dresses)
Choisis celle que Laura porte. -- Choose that (one) which Laura is wearing. (a specific one)

The tipoff is the relative pronoun que, which follows celle. Laquelle is by itself already a relative pronoun and doesn't require que.