This is an image of Mars. Click on image for full size version (40K GIF) Image from: NASA

Mars is small. Mars is about 1/3 the size of the Earth. This means that it cooled off very fast.

Mars probably started colder than the other earth-like planets. Then, Mars cooled rapidly from the outside, inward, like hardening candy. This caused the surface crust of Mars to rapidly thicken to the point of being immovable and prevented any further continental drift.

Much later, Mars finally warmed a little from inside. (Earth and Venus were already warm from inside!) Then a bubble of material rose from the deep interior of Mars, like the bubble in a tire, which pushed out and raised the crust, and created the Tharsis Bulge, Olympus Mons, and the other volcanoes.