Some people do name the notes from the top down, but I think most - in UK at least - do it from bottom to top; perhaps it's t'other way round in US. I think it's for the same reason that we describe a scale as do re mi fa sol la ti do, rather than do ti la blah blah blah.... In part music, the first part is the top - melody -line. So perhaps the numbering of strings follows this convention; the top, or 1st string supposedly being the melody string. I dunno, I wasn't there...
John Kelly.