So, to answer your question, good typesetting sets out to provide readers the most productive (rate of information processing for expended effort) reading experience that it can. For text: the caption summary is placed above the details to fit in with linear eye saccades and the pyramid principle of text interpretation; for graphics: the graphic is placed first to fit in with quite different attentional control mechanisms, non-linear eye saccading and non-linear information processing.
Another reason is that that we can generated the caption in the same chunk with the plot, if we do the computation before and only place the label after the processing (ie. the plot)
In the new system the plot caption is placed below the plot and table caption above the plot. Arguments for this are here: http://tex.stackexchange.com/questions/3243/why-should-a-table-caption-be-placed-above-the-table
The authoritative source is this one: http://www.amazon.de/Wissenschaftliche-Arbeiten-schreiben-mit-LaTeX/dp/3826691024 - Joachim Schlosser, Wissenschaftliche Arbeiten schreiben mit LaTeX, 3rd ed., p. 101
Another reason is that that we can generated the caption in the same chunk with the plot, if we do the computation before and only place the label after the processing (ie. the plot)