This is because the atomic radii of 4d and 5d transition elements are nearly same. This similarity in size $\left(\text{Z}\text{r}=160,\text{H}\text{f}=159\text{p}\text{m}\right)$ is due to lanthanoid contraction. Because of this contraction the radius of $\text{H}\text{f}$ becomes nearly equal to that of $\text{Z}\text{r}$ and as the properties of an element depends on its atomic radii they show similar properties.