Why didn't Russia just roll over Ukraine in a month, defeat their military and depose their regime, like the USA did with Iraq?

USA defeated the Iraqi military and deposed the regime in a month. Russia's most recent push on the city of Pokrovsk failed, in the 4th year of special operation.
What causes this difference?

USA defeated the Iraqi military and deposed their regime in a month. Why can't Russia do this in Ukraine?

>>512157707
Iraqi military was defeated and iraqi leadership was deposed in less than 2 months