"Scotland's Gift" will give you some insight, but, you make a mistake in assuming that the soil conditions on the entire North shore of the South Fork would have yielded better conditions and views.
Other sites, some by the Shinnecock Canal were considers, but, all in all, I think he selected a great site, especially when you consider that he wanted to DUPLICATE the great holes of the UK..
Finding sites for his Alps, Short, Redan, Bottle, Cape, Leven and others seemed to lead him to NGLA.