why does THF only function as a weak ligand for low oxidation state metals?

There are no AOs of a suitable energy (and symmetry) on the O atom of THF to accept π electron density from the filled metal d AOs. Indeed, you could argue that there would a repulsive interaction between the unbound lp on O and the filled d AOs on the metal. As it is O is hard and there is a poor H*OMO-LUMO match and the σ bond. M(0) is of course soft. So in the very useful Cr(CO)5(THF) the THF readily dissociates to form on addition of L Cr(CO)5L where L is soft ligand (PR3, π acceptor ligand).

M(III) -THF cmplxs are more stable.

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