As I see it, the selectors for any team (in any sport) will have a group of 'definite' players (whom they write onto the team sheet almost automatically) and a pool of 'additional' players (whom they use to fill the gaps).
Stokes is currently a 'definite' player (along with Root, Anderson, etc) but both Woakes and Curran are within the pool of 'additional' players. Although Curran was named Man of the Match in the First Test, it's Woakes who's achieved that accolade the most recently (including dismissing Virat Kohli, which must count for a lot), so that would seem to just give him the edge over Curran.
Woakes has picked up the first three wickets today, so he seems to be doing OK so far!