covalent bonding requries electrons to be 'shared' between atoms. In this case, they cannot share electrons between atoms because the numbers just do not match to make a full outer shell. They could in one way, but, it would take too much energy, and the atoms are too 'lazy' to do so.
Ionic bonding has the same story, they have to give too many electrons or take too many electrons. It just doessn't work
That is where the metallic bonding kicks in
Does that help?