why is nascent or atomic hydrogen more reactive than molecular hydrogen?

As the above gentleman said, nascent hydrogen has an unshared electron. Thus, it still seeks stability unlike the stable dihydrogen molecule. Nascent hydrogen would, therefore, be more interested in reacting than molecular hydrogen.

The atomic form has an unshared electron; in its normal state hydrogen exists as a diatomic molecule with a covalent single bond

Good discussion, just what I was looking for.