Caracalla made all people in his empire Roman citizens mainly to increase tax revenues

Evidence

Constitutio Antoniniana (citing Cassius Dio, Roman History Bk 78) (WikiProjectMed)

corroborated: 2 independent passages support this claim
XREF: connects to: Caracalla extended Roman citizenship to nearly all free imperial inhabitants — Stephen Kershaw, A Brief History of the Roman Empire, 1237; All free men in the Roman Empire were to be given full Roman citizenship — https://en.wikipedia.org/wiki/Constitutio_Antoniniana; Caracalla granted citizenship to nearly all free inhabitants except the small group of dediticii — The Text of the Edict (+1 more)

Caracalla granted citizenship to nearly all free inhabitants except the small group of dediticii (similarity 0.92)
All free men in the Roman Empire were to be given full Roman citizenship (similarity 0.92)
Caracalla extended Roman citizenship to nearly all free imperial inhabitants (similarity 0.90)
Universal Roman citizenship was enacted after citizenship lost its key privileges (similarity 0.87)
Roman citizenship remained non-universal after 212, excluding slaves, Junian Latins, dediticii, and barbarians (similarity 0.83)

Universal Roman citizenship was enacted after citizenship lost its key privileges #qualifies (0.70) — Stephen Kershaw, A Brief History of the Roman Empire, 1237