I photograph because I feel compelled to. I am hard put to indicate in words what the source of that compelling feeling is. I photograph with 12X20 for the same reasons that I photograph with 8X10 and 4X5.
I do realize that recently what I photograph is once again changing. I began some years ago by emulating the work of Adams, Fred Picker, and others. These were images of "things". By "things" I mean readily identifiable objects either of themselves or in relation to other objects as in a landscape. Recently I feel most strongly about photographing abstractions. I have been at this long enough that I follow my feelings, my hunches, and my inspirations.
As someone else has already stated the question "why" is impossible to address in language. Words are inadequate. The image speaks for itself. It speaks first of all to me and possibly and secondarily to others.