In short, the final outcome of the Yemeni war largely depends on who can better deal with the tribes.
And for now, the Houthis seem to be more effective in dealing with them, their methods ranging from regular deals to ransom and extortion (in some cases, the Houthis have threatened tribal leaders with revealing their hidden homosexuality or assassinating them).
Recently some Sunni tribes voluntarily joined the Houthis towards Lahij for an unknown reason (probably long term dissatisfaction with the PLC), which helped them a lot in the last breakthrough.