Dimir Midrange is full of countermagic (Counterspell, Force of Negation, Spell Snare) and discard (Thoughtseize), so resilience matters more than efficiency. The third Trinisphere further taxes their cheap interaction like Fatal Push and Spell Snare, making it harder for them to hold up mana for multiple spells in a turn. Giant's Boulder is lower-impact and your mana is better spent on resilient threats.