Why can the other projections be reprojected “on the fly” like the Mercator can?
For the Mercator projection, the reprojection to a given latitude of true scale is a simple matter of altering the reported pixel size and the upper left corner coordinate. For other projections the change is not as simple. They may require warping, which is beyond the scope of this extension. If you need to get two charts into the same projection, you would be better off using the chart reprojector tool mentioned in an earlier question.
