  Recently, the experimental discovery of high-$T_c$ superconductivity in
compressed hydrides H$_3$S and LaH$_{10}$ at megabar pressures has triggered
searches for various superconducting superhydrides. It was experimentally
observed that thorium hydrides, ThH$_{10}$ and ThH$_9$, are stabilized at much
lower pressures compared to LaH$_{10}$. Based on first-principles
density-functional theory calculations, we reveal that the isolated Th
frameworks of ThH$_{10}$ and ThH$_9$ have relatively more excess electrons in
interstitial regions than the La framework of LaH$_{10}$. Such interstitial
excess electrons easily participate in the formation of anionic H cage
surrounding metal atom. The resulting Coulomb attraction between cationic Th
atoms and anionic H cages is estimated to be stronger than the corresponding
one of LaH$_{10}$, thereby giving rise to larger chemical precompressions in
ThH$_{10}$ and ThH$_9$. Such a formation mechanism of H clathrates can also be
applied to another experimentally synthesized superhydride CeH$_9$, confirming
the experimental evidence that the chemical precompression in CeH$_9$ is larger
than that in LaH$_{10}$. Our findings demonstrate that interstitial excess
electrons in the isolated metal frameworks of high-pressure superhydrides play
an important role in generating the chemical precompression of H clathrates.
