IIS already uses all cores by default. What you are likely seeing is a single request using a single thread and thus a single core. This is by design.
If you make a 2nd request to the server, you should see that it hits a new core. With multiple users you should see even distribution across your cores.
If you need a single request to a single page use all cores then you'll need to specifically design it for that. That will require starting multiple threads to handle the single request. That's uncommon for IIS type load though that usually assumes dozens, hundreds or thousands of users rather than a single user gaining 100% of the resources of the server.