${\mathrm{SF}}_6$ is known but ${\mathrm{SH}}_6$ is not known because of

${\mathrm{SF}}_6$ is known but ${\mathrm{SH}}_6$ is not known because $\mathrm{F}$ being a strong oxidising agent can oxidise sulphur to its maximum oxidation state while hydrogen is a weak oxidising agent which cannot oxidise sulphur to its maximum oxidation state.