#2 Their last run was a failure

Love them, hate them, or ignore them - there is no denying that both of Shield's previous two runs were nothing short of a conspicuous failure. Their reunion in 2017 did not take off much since Roman Reigns had meningitis in the early part of the run and Dean Ambrose suffered a legitimate high-grade triceps tendon injury - rendering him out of action for around six months.
Their reunion in 2018 was even more disappointing since the primary reason for such a reunion was to protect the newly crowned Universal Champion, Roman Reigns, from hostile crowd reaction.
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On the RAW after SummerSlam 2018, while Braun Strowman was attempting to cash in his Money in the Bank briefcase on Roman Reigns, Ambrose and Seth Rollins came to rescue their brother. However, when Reigns was diagnosed with cancer, WWE had to drop that plan as well.