The reason for greater range of oxidation states in actinoids is attributed to:

Unlike lanthanoids, where the $4f$ and $5d$ subshells are separated in energy, the $5f,6d$ and $7s$ subshells in actinoids have very small energy differences. This allows electrons from all these subshells to participate in bond formation, resulting in a wider variety of oxidation states (up to $+7$ in some cases).