Idk, why are Putin and Kim Jong Un still in power
It's all about system with checks and balances.
Presidential form of government and single party system are "more prone" to dictatorship as power and authority gets centralised in one person or party, if institutions aren't well equipped to counter it.
A leader with authoritarian tendencies will start with state capture - appoint loyalists on key positions in various institutions, fire those who dissents or oppose, suppressing freedom of speech and so on.
A national narrative based on fear is build and people fall for it.
Even the rich would remain rich and free if they stay loyal and it happens through back stage deals.
Information would be controlled.
Gradually, it extends to opposition and elections to stay in power.
Trump's President rule isn't just exposing Trump but also exposing the institutional flaws in the US system of governance