Why is there a difference in mechanism between tertiary halogenoalkanes and primary halogenoalkanes in nucleophilic substitution?

The difference in mechanism between tertiary halogenoalkanes and primary halogenoalkanes is caused by the difference in relative stability between the two carbocation intermediates formed. In the case of tertiary halogenoalkanes which have methyl groups attached to the functional group carbon, the methyl groups are able to stabilise the positive charged formed upon nucleophilic attack on the functional group carbon, making the intermediate more stable. This is not seen in primary halogenoalkanes, where the inductive stabilising effect of the functional group carbon is only caused by one methyl group as opposed to three. This is why tertiary halogenoalkanes follow Sn1, and primary halogenoalkanes follow Sn2.