Why do isotopes have different chemical reaction rates? Isotopes of a given element differ in the rates at which they undergo chemical reaction.

Why is this so? The only explanation I could think of is that the gravitational force on the electrons increases and hence the electrons are better bound to he nucleus. But this force is very negligible.
Is there perhaps another contributing factor?
Isotopes differ in mass.
Otherwise the same molecules but with the different mass have the different mean speed.
The different speed means the different molecular collision rate.
The different collision rate means the different kinetic constants.
There are some indirect effects of different isotop mass on the kinetics, but these are much smaller than the direct kinematic effect.