Is it not obvious? Because if we were adequately able to listen to and understand his concerns, we may have been able to find the necessary reassurances or compromises which both sides could have lived with, which might have prevented this in the first place. Is it so inconceivable that had - for example - Ukraine agreed to not join Nato for say 10 years, we could have averted this for perhaps 10 years? Maybe Putin would not be around by then.
Anyway that ship has sailed, massively to everyone's detriment. But I keep being told by those who seem to think they know everything that his next step is Poland or some other non-Nato country. I don't believe it is any more, if indeed it ever was. But it might be a bloody good idea try to figure that out, rather than just assume that he's bonkers and that's all there is to it.