WWE consciously chose not to bring two stars to the company, at least until Roman Reigns lost. As a top WWE name noted, they then had no control over what happened next.
Wade Barrett made a major revelation on commentary this week. On SmackDown, with no Solo Sikoa, after having lost his Tribal Combat match against Roman Reigns last week, the rest of the Bloodline were free to do what they wanted. While officially not the Bloodline anymore, Tama Tonga and Jacob Fatu took matters into their own hands for large parts of the show this week, attacking several stars and making their presence known.
As Sikoa is no longer the Tribal Chief, he is no longer there to control them. On commentary, Wade Barrett revealed that this was why WWE had not brought Tama Tonga or Jacob Fatu into the company. He said that they had been consciously kept out.
However, when Roman Reigns lost his match against Cody Rhodes at WrestleMania 40, Solo Sikoa launched a coup and took over. He said they were uncontrollable, and the company had been afraid of that.
It remains to be seen what happens next with the two stars. When Solo Sikoa returns, he may be able to take them in hand again, but for the moment, they seem set on rampaging.