What was the main reason the Polish–Lithuanian–Muscovite Commonwealth never materialized?
Lack of resources
Incompatible demands from both sides
Natural disasters
Lack of support from the populace
Answer
The main reason the Polish–Lithuanian–Muscovite Commonwealth never materialized was due to incompatible demands from both sides. This means that the Polish–Lithuanian Commonwealth and the Czardom of Russia could not agree on the terms of the union.
From Union to Division: Exploring the Polish-Lithuanian-Muscovite Commonwealth