1) Why U over R, W or G? (And why ever R, W, or G?)
So first, why you would run each color mainboard:
White mainboard focuses on either removal running mainboard Dispatch or in Cranial Plating "combo" kills, via Steelshaper's Gift.
Green mainboard gives you Collected Company typically as a one of. This helps to provide greater redundancy, as well as a more dynamic bounceback from blowouts.
Red Mainboard gives you Galvanic Blast, which gives you either reach or removal. Back when Twin was the deck to beat, you pretty much had to be on R affinity.
Blue Mainboard gives you Thoughtcast, which gives you more gas, and allows for better recovery from blowouts.
A quick note on Black Mainboard: Don't (outside of Vault Skirge and Cranial Plating of course). It's pretty much always reactive, which is not where you want to be mainboard with affinity, unless you have a really really combo heavy meta, and really need Thoughtseize main, and even then it's still usually not your best option.
So why U? Because 1) it's the fastest variant, outside of R, but still has good recovery potential. With K-command and friends running around in mainboards you need to be able to recover from losing a key piece or two, and U and G offer this, however U does it for less mana (normally). W for Steel Shaper's Gift often just leads to larger blowouts to removal/disruption by sacrificing threat density for greater reliability in Cranial Plating kills. Additionally, it's just generally slower than you would want, requiring 4 mana total investment to get a single threat. While W for Dispatch is focusing on removal over reach or gas/recovery in a deck that prioritizes speed, which just isn't usually something you need/want to do in the current meta. For reference, many decks right now run a combination of U and R.
2) Why 4 Etched Champion? Firstly my local meta has a lot of Control, and Etched Champion is an absolute house in those matchups. Additionally Etched Champion is great against Death's Shadow decks (it blocks Shadows for days, then can swing for unblockable lethal), and races better than a card like Steel Overseer.
3) Why only 2 Steel Overseer? Steel Overseer is great against Zoo decks, and the like, and those have taken a beating recently, so there are fewer decks the card is great against. Additionally the card is just often too slow, sometimes if grants you what you need to win, but it doesn't do anything the turn it comes into play, and even E-Tron can relatively-easily deal with its 1 toughness body, before you can get an activation off. Essentially, the decks this card is good against aren't very good right now, making the card not as good. (And for reference, while CoCo decks can be similar to Zoo decks, they don't present the same kind of size threat that zoo did, especially the new Devoted Druid variants )
