The EPIC Storm is a combo deck that wants to resolve Beseech the Mirror or Echo of Eons and chain rituals into Tendrils of Agony for the kill, so proactive disruption is essential since we have little relevant permanent-based interaction against their spell-based combo. The extra Thoughtseize lets us strip a key piece like Lion's Eye Diamond or their combo payoff before they can assemble it, especially since Veil of Summer can protect them from our first copy. Fatal Push is mostly a sideboard inclusion to pick off a Hexing Squelcher, which can make their spells uncounterable and protect their combo pieces. Sylvan Safekeeper and a fourth Wight of the Reliquary are cut since they do little to stop a combo kill.