As the title says, what are the arguments usually made by scholars and non-Śāktas (Vaiṣṇavas & others) when they say the Devī Bhāgavata Purāṇa doesn't belong in the list of 18 Mahā-purāṇas?
The scholarly opinion (R. C. Hazra's) is that it was written in the 11th or 12th century A.D. so it cannot be a Mahā-purāṇa.
The Devī-Bhāgavata, though a work of the eleventh or twelfth century A.D., claims to be the real Bhāgavata Purāṇa and includes the Vaiṣṇava Bhāgavata among the Upapurāṇas. But it really is an Upapurāṇa adapted carefully by its author to the description of the Bhāgavata Purāṇa contained in the Matsya, Agni, Skanda, and other Purāṇas. An examination of this work shows that its author was a Smārta Śākta Brāhmaṇa of Bengal, lived for a long time in Banaras, and then wrote this work for infusing Śākta ideas into the members of different sects by adapting it, as far as practicable, to the views of these sectaries.
(The Cultural Heritage of India, Volume II)