One could say that the scene at that time was really just like the situation today. Although it was not as dreadful as it was said to be, for these weaker Mortal Clans, they truly did not reject the idea of integrating into the human race.
"The moment one turns to humanity, the heavens and earth instantly become vast." That's exactly it.
For their own feeble clans to merge into the human race and grow stronger together, to let themselves and their descendants no longer be inferior, to stand proudly between heaven and earth, these weaker clans harbored no reluctance at all but yearned for it instead.
With the addition of these weaker clans, the force of the Suiming Empire became even more formidable, especially as the surging Qi Luck of the races proved their loyalty, which greatly delighted Sui.