Yes, there is a reason: because those DCs are not for the saving throw “against spells” as indicated by Spell Focus. The “Difficulty Class for [...] saving throws against spells” has a definition:
A saving throw against your spell has a DC of 10 + the level of the spell + your bonus for the relevant ability (Intelligence for a wizard, Charisma for a sorcerer or bard, or Wisdom for a cleric, druid, paladin, or ranger). A spell’s level can vary depending on your class. Always use the spell level applicable to your class.
(Spell Descriptions: Saving Throw)
Note the usage of the exact same wording used by Spell Focus. This is the DC that Spell Focus modifies.
If a spell description references any other DC—any other number—then that is definitionally a different DC from the spell’s DC (or else they would be the same, not different). And Spell Focus only increases the DC for “saving throws against spells,” which again, is this one, not another one. The DC 15 Reflex saving throw in firebrand is not a save against the spell, it’s a save against an ongoing fire damage condition—a condition caused by the spell, but which does not use the DC for the saving throw against the spell, which means it does not use the DC that Spell Focus actually improves.
This is basically the same as trying to use Spell Focus to improve the saving throw DCs of effects used by summoned creatures, and it doesn’t work for exactly the same reason.