Now, I don't like guessing. When Russia entered negotiations the war was clearly not going its way, so the obvious explanation is that Putin wanted to disentangle from it with the least damage to his image as possible.
The question is why the negotiations failed, if they were really as advanced as suggested in the articles I linked above. Since the name of Boris Johnson has been brought up (in the context of being one of the Western representatives that told Zelenskiy to drop the deal) I tend to believe that a peace deal failed because at least one side in the negotiations was incompetent fools who should have never made it in power. That the other side is probably the same makes no difference.
The problem is that Western media picks too easily up even the most stupid Russian narratives.
To distract western military support.
To distract offensive\defensive ukrainian plans.
To construct a peaceful narrative at home or the war mongering west.
To strengthen or pet allies, who mediate.
To gain intelligence about the willingness of territorial sacrifice.
Would Putin ever publicly admit his grand soviet plan? If no, then how would he behave instead? Consider georgia, chechenia, belarus, etc.