If Rezen has to guess the reason, it would be that the City of Hope is currently lacking manpower and they wish to replenish their numbers by absorbing the scattered tribes
In fact, calling the tribal people manpower might not be accurate. Maybe, the more proper term is cannon fodders depending on how bad the situation was
However, even then, Rezen already plans to become a part of this city. A developed society with resources, knowledge, and instructors is basically what Rezen badly needed
He is still lacking in all three and his only hope in this realm is the City of Hope as it was the only human city in the realm
"I understand. I will help you persuade them, trust me!" Rezen said boastfully, once again portraying a cheerful personality
It was easier to love and have goodwill towards someone that is cheerful, energetic, and sunny rather than those that are gloomy