well, that is a thing you will get in practicing Arabic and to answer that correctly I googled that to make sure because we don't discuss that issues in our learning generally the main idea of that is: the vowel ي in فيهم , إليهم , عليهم etc. made the هـ sounds with كسرة and even if ي was hidden like in : يكفِهم we say it with كسرة which was يكفي + هما and we get it combined and cancel the ي because the word is مجزوم like this sentence " أولم يكفهم " if it wasn't مجزوم we say it يكفيهم and still the هـ sounds with كسرة and from that, we can say the vowel ي is making the كسرة after him you can say that is: there is no major reason and it is all about how the Old Arabs used to say it 1500 years ago if you see the word منهم it sounds in ضمة because there is not ي in that word you can use http://www.almaany.com/ for making sure of any word vowels and meaning

well, that is a thing you will get in practicing Arabic and to answer that correctly I googled that to make sure because we don't discuss that issues in our learning generally the main idea of that is: the vowel ي in فيهم , إليهم , عليهم etc. made the هـ sounds with كسرة and even if ي was hidden like in : يكفِهم we say it with كسرة which was يكفي + هما and we get it combined and cancel the ي because the word is مجزوم in this sentence " أولم يكفهم " if it wasn't مجزوم we say it يكفيهم and still the هـ sounds with كسرة and from that, we can say the vowel ي is making the كسرة after him you can say that is: there is no major reason and it is all about how the Old Arabs used to say 1500 years ago if you see the word منهم it sounds in ضمة because there is not ي in that word you can use http://www.almaany.com/ for make sure of any word vowels and meaning