They are both noble gases.
To me it feels slightly wrong to say that they don't easily bond, that suggest that they try but fail. They don't want to bond with other elements so they don't try.
All an element wants it have a full outer shell, 8 electrons. Randon and krypton already have 8 electrons on their outer shells, so they don't need to bond with anything else to make up the numbers
When an element bonds covalently, they are sharing electrons to get up to 8. When they bond ironically there is a transfer of electrons from one atom to another, so they can get a noble arrangement.