The Why? of contingent existents

The discussion continues at MP.   Bill says the question cannot be answered by invoking contingent beings as explanantia.  He also says that 'concrete contingent being' (CCB) is not a sort or kind.  Rather,  contingency and concreteness are modes of being.  But if this is so, what do CCBs have in common that a general explanation could possibly latch on to?  Surely, their very contingency calls for a local, particular, explanation, in each case? 

Commenter Steven Nemes offers an interesting analogy with  'burning things'.  Steven says that it is not good enough to say that the cause of something's burning is another thing's burning.  This does not explain why things burn at all.  Steven appears to be asking for an explanation of flammability---why some things burn and others do not.  That's a reasonable question.  But the analogy with existence breaks down at a critical point.  For we all agree that everything exists and nothing fails to exist.  So there is no contrast between existence and non-existence for any explanation to account for.

I'm wondering if the emphasis on contingency hasn't led us astray.  Why not frame the question in terms of material objects?  Presumably every material object is concrete and contingent?  At least then we would have a kind in the neighbourhood that a general explanation might address.  And lest 'material object' be thought a 'dummy sortal' and hence open to Steve Maitzen's countability argument we may simply exclude parts of wholes as objects in their own right.  If I take the cap off my pen I have two objects.  Put it back on and I have one again.

Aside:  in a comment Bill explicates 'contingent' as 'possibly such as not to exist'.  I guess Bill sees this as a 'modal property'.  He has referred, I think, to 'complex modal properties' elsewhere.  I really worry about this sort of language.  I'm beginning to think (no, I've thought for a long time) that contemporary analytic philosophy gets modality completely wrong.  But this is a huge topic and any ideas I have on it will probably remain inchoate.