Dante restricted the flames of hell to particular sins, rather than imposing them homogeneously to all faults. The worst sins of pride and betrayal are punished by encasement in eternal ice. Those who once acted with hot fury against love are now permanently immobile in their sin.
There is also the symbolism that, in Dante’s medieval cosmological view, the earth was the farthest point from the Empyrean, the dwelling of God. The center of the earth would be the utmost point from Heaven’s eternal flames of love, and thus it is the coldest place in the cosmos.