There was an excellent analysis on that negotiation, it's a good paper and discusses the reason for the breakdown of the negotiations and included:-

A final agreement proved elusive, however, for a number of reasons.

Kyiv’s Western partners were reluctant to be drawn into a negotiation with Russia

The public mood in Ukraine hardened with the discovery of Russian atrocities in Irpin and Bucha.

They tried to deliver an overarching settlement even as a basic cease-fire proved out of reach.