Credibility of democracy breaks down as you scale upward (which you have to do if you want to centralize). Any representative democracy in which the representative doesn't know all the people they represent is already suspect, but when you get to the point where a single guy supposedly represents hundreds of thousands or even millions, it's kinda obvious that there's no meaningful representation involved. The only way to avoid that is to grow the parliament instead to the point where it ceases to function as a deliberative assembly (and then what's the point of it?).

Or you can have a bunch of smaller assemblies that actually are representative, and then a larger one to which assemblies delegate their own to cooperate. But that's exactly political decentralization - a multi-level federation.