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Why did Pope Gregory IX excommunicate Frederick II in 1227?

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Pope Gregory IX excommunicated Frederick II on September 29, 1227, primarily due to the Emperor's repeated delays in fulfilling his long-standing vow to lead a Crusade.12

While Frederick had finally launched his expedition from Brindisi in 1227, an outbreak of plague among the Crusaders forced him to return to port. Pope Gregory IX, who had grown increasingly impatient with Frederick’s previous postponements and suspicious of his sincerity, dismissed the Emperor's justification for the delay. Additionally, tensions were exacerbated by Frederick’s efforts to consolidate power over both the imperial throne and the Kingdom of Sicily, which the Roman Curia viewed as a threat to the church, as well as allegations regarding the abuse of rights of Sicilian churches during papal vacancies.132