s1. That is one reason for it being so difficult to ensure any object is completely free from any kind of living organism.

I happened to meet this sentence.
Though I can understand it roughly, but I can't grammatically completely.
Is 'for' used as a preposition or a conjunction?
Is 'being' used as a gerund or a present participle?
It can also be understood like "it is so difficult to ensure ~."
But I have no idea why the verb 'is' should be turned into 'being'.
I've never seen this structure before.
Please make it understood.
Thanks in advance.