"Reluctant?"
"Of course I'm reluctant."
Even though oppressed by the Righteous, the Six Demon Sects are still Great Sects, not like those small Sects that practice Demon Techniques or the Loose Cultivators who are easily killed by passing Swordmen, becoming merely a notation in the record of a Swordman's acts of chivalry and righteousness.
Just look at the Yin Demon Sect, behind almost every brothel in the world, there are shadows of the Yin Demon Sect. Royals and nobles might very well have a wife or concubine who is a member of the Yin Demon Sect.
And then there's the Demon Sword Dao, a Sword Dao with inversion at its core. All adversities in the world serve as nourishment for the Swordmen of this arrogant Sect. How could it possibly submit to the Heavenly Demon Palace and be integrated by it?