#2. Failure to build Rollins up
Usually, WWE uses the two months between Royal Rumble and Wrestlemania to build the Royal Rumble winner up. This step is even more valid when the Royal Rumble winner has to face someone as huge as Brock Lesnar.
In 2015, Roman Reigns won the Royal Rumble to gain the rights to challenge Brock Lesnar at Wrestlemania 31. However, WWE had to build the Big Dog as a credible challenger to the Beast. What did WWE do then? They pitted Reigns against Daniel Bryan - the most over babyface who was incredibly protected at that time - at Fastlane 2015.
Reigns defeated Bryan clean and even kicked out of the Running Knee. After that match, Reigns looked like a proper challenge to Lesnar.
However, this time, WWE took no such measures. At Elimination Chamber, Rollins had no role, and at Fastlane, Rollins became a side character in the Shield attire.