Hazard a guess, it's pretty simple. Either:

A) their marketing email system not only doesn't have dynamic access to their frequent flier balances, it doesn't even have access or code to know what people already purchased from the miles partners from the same offer,

or

B) they don't have the time, money, or competence to get someone to think about the logic that should say "don't offer to someone if miles_purchased_2019 > max_dollars"