Sometimes the brand is cheaper. That is happening right now with Retin-A and generic Tretinoin.

Sometimes the generic actually has enough of a difference, or a specific difference and it doesn’t work as well (Imitrex vials vs Sumatriptan from India - often I have to use 2 of the generic, never had to with the brand, I still but the generic because price wise it’s less than half the cost but there is a noticeable difference)

If they don’t have a reason I have no idea, and guess you can’t either. But those are the two rational reasons I can think of.