Why was Urizen so strong?

[GetToTheTop]

Besides plot of course.

[Sansei Muramasa]

Urizen was so strong was because plot plus he was basically the purest form of Sparda's next generation.
It's as close as Sparda as you can get.

Sparda was one of the strongest demons in existence we all know that.

[FE-RN]

Urizen at first was probably as strong as Dante and Vergil (pre-Mundus corruption). With the Yamato in hand, he was strong enough to obtain and control the Qliphoth Tree. Once Urizen had the tree, he started to use demons to collect human blood to power up and grow the tree. He connected himself to the tree thus whatever power up the tree received from human blood, Urizen received it as well. Also finally he reached peak power when Urizen ate the blood fruit. However, Urizen was weaker than SDT Dante due to the fact Urizen was missing his human side. Once Vergil was reconstructed, he was able to access his own version of SDT due to the blood fruit and not using the same method Dante used.

[Ziron]

he is also PURE demon where they are only half demon

[PJ]

Urizen was strong while he was sitting on the chair because he was jacked into the tree and it was just pouring power into him. Once he was disconnected from the chair he started getting weaker. By mission 17, even after eating the fruit, he's way weaker than he was in the chair which is why that boss fight is one of the easiest in the game.

It was all the tree man! Urizen on his own is kinda junk.