Early interaction, creatures, planeswalkers that demand answers, and a solid late game make the perfect formula for a winning midrange deck. U/R Midrange offers all of those elements in spades.
The Izzet colors are flooded with early interaction, especially for creatures. For just a single mana, Magma Spray can handle many of the important aggressive creatures in the format. Exile is extra important against eternalize and Scrapheap Scrounger, which could otherwise be a real issue for a midrange deck. At 2 mana, you’ve got Harnessed Lightning and Abrade. Harnessed Lightning kills small creatures and can scale up well in the mid-to-late stages of the game. Abrade gives you answers to Gearhulks, Vehicles, and God-Pharaoh’s Gifts. Fiery Cannonade can give you an instant-speed way to clean up after small creatures and tokens.
Most of the best threats in Standard are so good because of how hard they are to answer. There are indestructible creatures, creatures that come back from the graveyard, and plenty of insane enters-the-battlefield triggers. Countering them usually saves you the trouble, and Essence Scatter and Disallow give you some hard answers to problematic threats.
Whirler Virtuoso is a great way to stem the bleeding against aggressive decks or help go wide against control decks. Whirler can use some of the extra energy this deck can produce for Thopters and produce multiple threats or answers depending on what the matchup calls for.
Glorybringer is what Standard is all about. Glorybringer requires an instant-speed answer, which also means that the opponent needs to have their mana available at all times. Even a single combat step with Glorybringer is often enough to swing a game as the exert ability is so powerful.
Torrential Gearhulk provides your late-game plan, giving you additional counters, removal spells, and card draw, all attached to a massive creature with flash. It’s good enough for Modern and Vintage, so it’s no surprise that it’s so strong in Standard.
Glimmer of Genius provides card advantage to get ahead while finding the threats and answers you need. Flashing it back with Torrential Gearhulk provides extra value, and the energy that Glimmer provides can pump your Harnessed Lightnings or make Thopters with Whirler. Search for Azcanta doesn’t provide immediate card advantage, but the card selection early is definitely useful. Once you get to the middle stages of the game, you get to flip your Azcanta into another mana source and a card advantage machine.
Chandra, Torch of Defiance was the most hyped planeswalker since Jace, the Mind Sculptor. It’s so heavily played in Standard today that we’re all plenty aware of what a mana producing, creature killing, card advantage engine is capable of. Saheeli Rai, however, isn’t one you see often, especially since the Felidar Guardian banning. Saheeli is a cheap planeswalker that can tick up to reasonable loyalty and give you some card selection. Saheeli also combos well with all of your creatures, making more Thopters with Whirler, flashing back more spells with Gearhulk, or just creating another Glorybringer.
There are a lot of ways to build successful midrange decks in Standard, and U/R has a great combination of powerful creatures, strong planeswalkers, and cheap interaction!