Eldrazi Tron's whole plan runs through Urza's Mine, Urza's Power Plant, and Urza's Tower producing extra mana together, plus Eldrazi Temple and Ugin's Labyrinth, so Damping Sphere is devastating, capping every one of those lands at a single colorless mana. Molten Rain destroys a nonbasic land (nearly all of theirs are) for a huge tempo hit plus 2 damage. Smash to Smithereens blows up Expedition Map or Mind Stone while dealing 3 extra damage to close the race faster. Eidolon of the Great Revel is unnecessary risk here since their deck is slower and less spell-dense, but the raw damage output of Molten Rain and Smash to Smithereens matters more than the marginal pings.