Probably because of concerns about Story's arm. The team may be like 80% confident that it will work out, but without actually putting it to the test in actual game situations there's at least some possibility that the arm issue will make it non-viable. The team looks worse if they announce they are fully committing to Story at SS and then it turns out in Spring training that it's not going to work out. It's going to look bad either way, but it'll definitely look worse if the team expresses full confidence in Story as a plan and it turns out to not be viable.
Story is also a big enough name that if they commit to him publicly and then manage to swing a deal for another starting SS it'd risk alienating Story.
I suspect the plan is to pursue opportunities to acquire a starter, but if that doesn't work out Story will be plan A with Hernandez or filler as plan B.
Now why don't they say that publicly? I'd guess because it looks like a vote of no confidence in Story, and in the event that the team wants to try Story at short they'd rather go into that experiment with as clean a slate as possible.
As a fan it's frustrating, but I can see why they are doing it especially given the outright hostility they are getting from the fanbase and media. Without commenting on whether I think that hostility is earned, I think it's easy to understand why they'd keep mum if they expect anything they say to be interpreted in as negative a light as possible (and let's not pretend that isn't exactly what would happen).
