Running horizontal at the top of the page just takes up more of the already limited vertical space. Also, we're used to time "running vertically" from the history listing, so a tool with time running vertically fits in better with how our brains work! :-)
Topic on Extension talk:RevisionSlider
Myself, I do not see this as an issue. Horizontal line is clear and interaction with it (seems to me) simpler than it would be with a vertical line.
The functionality is superb, congrats Wikipedia team.
I, too, would much prefer a vertical presentation of the graphical data. (or, an option to select one or the other as user-selected default). ~~~~