Perchloric acid is strongest acid, but weakest oxidising agent. Why?

HCLO₄ has ${\mathrm{pK}}_{\mathrm{a}} <\hspace{0.17em}-10$. Its ability to provide proton is highest.
This is due to resonance stabilisation of perchlorate ion. ${\mathrm{CIO}}_4^{-}$ has four cannonical structures and six delocalised pi electrons.
${\mathrm{CIO}}_4^{-}$ is resonance stabilised. It does not furnish oxygen and hence a weak oxidant.