cool idea but I'm gonna assume its a lot more expensive to send up an X-37 and have it crash back down then it is too just send a few nukes at your target.
If you look at it purely from a costs perspective, maybe.
Don't discount the PR disaster that would ensue from the US using nukes in any scenario other than retaliation for a nuclear attack. Nukes used in anger would likely mean worldwide condemnation, sanctions, and pissed allies complaining about fallout.
Preface: I don't think this is a likely explanation.
But if you think it is, there's a big advantage of kinetic weapons over ICBMs, namely that they are a lot more credible as a first strike weapon. ICBMs launches are quite detectable, allowing counter strikes (hi there nuclear deterrence).
Don't discount the PR disaster that would ensue from the US using nukes in any scenario other than retaliation for a nuclear attack. Nukes used in anger would likely mean worldwide condemnation, sanctions, and pissed allies complaining about fallout.