I'm hoping it's a trap for the Dems. By holding on to the files, I think the Bush admin is trying to generate a lot of noise to have these files released. My hope is that the information is particularly damaging for the Clinton administration. To release in an election year, even if it's the Commission calling for it, it might still look like a partisan attack on Clinton and the Dems.
By trying to hold on to them until they cave under political and public pressure, they remove the appearance of partisanship that tainted the Monica "it's only sex" case and various rape charges/investigations that the Republicans presided over.