Because the First House is voted into office, the Second House is comprised of military and business leaders who'd take exception to being killed off wholesale and the Third House is voted in by the First and Second Houses.
And while it's entire possible he could coerse the military and the public, the business leaders interests could never be served by allying themselves to a power-mad dictator and they could topple the entire economy if he ever tried to move against them, leaving the country in total chaos.
The system balances military power, economic power, the will of the people and the honour of patriots to ensure that while the Dictator can rock the boat, if he ever tries to make himself the de-facto captain the entire ship sinks killing everyone aboard.