Actually, Apple blocked the Steam remote play app because it just acted as a remote desktop for Steam's Big Picture mode, which also allowed using the store for making purchases. This was a thorn in Apple's eyes so Valve had to ensure the shop was inaccessible when opening Big Picture mode through the iOS / iPadOS app.

Source:

- https://www.theverge.com/2018/5/24/17392470/apple-rejects-va...

- https://www.theverge.com/2018/6/5/17430532/apple-app-store-r...

Steam is more important for gamers than iCloud. Not having that means losing many of those gamers to Android. Perhaps they hope that those gamers will buy some games from Apple Store eventually.