Why not "some reasons" but has to be "some reason"?
I think like this.
The determiner "some" has two meanings/functions -- one is to imply a multiplicity of things and the other is to kind specifically mention or identify one as it is being mentioned or being referred.
Examples
Give me some apples. -- implying a multiplicity of things/apples.
'If you are worried about some aspect of your children's health, please do not hesitate to call us. -- here the determiner "some" is being used to refer to something in particular.' this is the sense in which 'some' is used in ' for some reason' - it is an idiom and it isn't used with reasons. Of course, in other contexts 'some reasons' - meaning several reasons - would be correct. For example: L