The problem is that only store apps are allowed to be installed on Windows RT devices, and porting Chrome to a store app is not an insignificant amount of work,

The problem is that only store apps are allowed to be installed on Windows RT devices, and porting Chrome to a store app is not an insignificant amount of work, for what amounts to very little gain (RT customers would be the only ones who benefit). There's an argument that future store-compatible apps are easily ported to WP10, but Google has very little incentive for any of this work. If anything Mozilla had reasons to, but even they killed their efforts due to lack of customer interest.