The sentence "People raised in Spain and Mexico don’t use exactly the same form of Spanish." is not appropriate for an academic essay primarily because "It includes a contraction."
In academic writing, it is generally preferred to avoid contractions (like "don’t") in favor of the full form ("do not") to maintain a formal tone. While opinions and topic breadth are important considerations in academic writing, the primary issue here is the use of informal language indicated by the contraction.