The reason why a centralized system dependent on the Emperor was formed was because the Republic faced a security crisis. The Senate was asked to respond quickly to the clear threat.
The Congress was divided on whether to have a regular army, but they unanimously passed the emergency powers.
Without proper verification or fact-checking, they gave a lot of power to one person in just one day.
As you know, the country transformed into a dictatorship as the war became protracted and bogged down.
After the collapse of the Empire, the New Republic made it difficult to exercise emergency powers in order to prevent the creation of a dictator again. Giving a lot of power to one person, even temporarily, is inappropriate and too risky. The New Republic has revealed that Palpatine was the mastermind behind the creation of the crisis and that it was a Sith conspiracy. Based on numerous testimonies.
The Republic boldly announces the fact that it had been under the control of the Sith for many years and that even the Jedi had been deceived.
The Emperor was dead, so it could be made public.
Some believed it, others didn't, but it did dispel the suspicion that the Jedi were plotting to overthrow the Republic.
The New Republic tried hard to prevent a second Emperor from being born, but they were working on the assumption that a security threat would never occur again. However, in theory, if a security crisis really did occur again, the New Republic would still be vulnerable.
They hadn't thought out a real solution.
They didn't think of Palpatine and Darth Sidious as separate entities.
It is true that the Emperor was a sinful Sith Lord, but if we consider Chancellor Palpatine as just one person, he did nothing illegal. The Senate at the time accepted the Chancellor's emergency powers, and the Chancellor always had to be approved by the Congress during wartime.
In other words, it was the Senate itself that created a dictator.
If you don't know Palpatine's true identity, you can seriously think of a real solution, but as soon as you know his true identity, you stop thinking. The New Republic made the facts public, so many people found out his true identity. The Emperor deceived them by having them reveal his true identity.
As a result, a New Republic was formed that was weak in a crisis situation.
There was a complete lack of consideration for security, which was exactly what the First Order wanted.
They rejected centralization and had many security amateurs at the leadership level.
The First Order's invasion made the threat a reality, and they easily surrendered.
"The New Republic is weak because it cannot exercise emergency powers, and an invasion will be easy."