offence against satellites is relatively easy (a bucket of bolts in the right orbit)
The nations that have the capability to launch into the upper orbits all have enough dependencies on their own orbital resources to not be willing to loose them.
The nations and organizations that are willing to burn everything down as long as they take their enemies with them do not have the capability.
they are working harder to get Nukes because Nukes don't require the infrastructure to deploy, you can deploy a Nuke in a truck, boat, etc Plus people have an unreasoning fear when Nukes are mentioned so they make good terror weapons.
As for speed being critical for space based attacks, that depends on the situation. It doesn't matter if an asteroid is redirected in days, weeks, or even months if it's going to take months for anyone to get to it and be ready to redirect it to go elsewhere.
If you think an aircraft flying into a building at a few hundred miles an hour is bad, it pales in comparison with something hitting at even low orbital velocity (remember, energy is M*V^2), at those speeds you don't _need_ a warhead.
When you read about the hypervelocity weapons the military is working on, this is exactly what is going on. They are working to get missiles that are fast enough that no warhead is needed (and they have the side effect that even at long range, the chance of being able to intercept is poor)
It is well within the capability of the satellite launching nations to build space-based weapons that would be extremely powerful, but all these nations have agreed not to go there, and so far none of them has been pressed enough violate (or at least to admit to violating) that agreement.