Why is there no share count on my Stack buttons in the pinned bar?

Because in Stack the count would be a third line under the name rather than a word beside it, and the pinned bar tells the rest of the page how tall it is before it draws anything — a taller bar than the page was told about would sit over the last lines of the post. So in Stack the count shows in the flow of the post, where there is room for it, and not in the floating rail or the pinned bar. Every other style shows it everywhere, because there it costs width rather than height.