Thats because proliferate doesnt synergize with any specific card type, mechanic or game action: it has synergy with the way card designs get translated into Magic rules text
Despite not offering any additional ramp or added abilities, the Dual Lands in MTG are all just simply good
The amount of life you lose is determined by the permanent's mana value as it last existed on the battlefield
While Shatter the Source does fit into this deck much better than Wrenns Resolve, as it has Convoke, it is still simply an awful card
Grave Pact Grave Pact does a lot of work in the typical black sacrifice decks, and it also combos pretty hard with popular Golgari commander, Glissa, the Traitor
2 Fires Of Invention Was Easy To Work Around Fires of Invention tried to balance its powerful effects (casting spells for free if a player's mana cost was less than their number of Lands) with its powerful drawbacks (a maximum of two cards could be cast in any way, and the player could only cast spells during their turn)