Izzet Delver runs a critical mass of free counterspells (Force of Will, Daze) to protect its clock, so Defense Grid making those cost an extra {3} off their turn is a huge roadblock to resolving Aluren or Show and Tell safely. Veil of Summer is there to make your spells uncounterable for the turn, and it draws a card if they've cast a blue spell, which Force of Will, Daze and Spell Pierce all are. Omniscience is too mana-intensive against a deck that can race quickly with Dragon's Rage Channeler and Murktide Regent, so it's cut along with one copy of Atraxa, which is also a slow, expensive play into their countermagic and fast clock.