When I travel by train to London, it is usually for work. My work's travel policy is that I should buy the "cheapest standard class fare for the journey I wish to make", with a further note that "fares should be purchased two weeks in advance where possible".
I suspect most, if not all, companies are the same. Following on from this, it would seem to me that First Class (*not including Weekend First) is aimed at either Executives, or those people willing to pay for a more luxury product. Given that a lot of people on the Newcastle - London route are tourists, hen/stag parties, students, and people travelling for work, I can certainly see there being more demand for standard than First on any given service.