Such maps are typically made in Adobe Illustrator. The use of similar markers and colors is merely a convention that authors are used to. In addition, the vast majority of K-12 textbook maps are created by only three or four design firms.

Modern mapmaking often makes extensive use of GIS software like ArcMap or QGIS, but that's not all that useful for historic cartography, since the available datasets typically show modern roads, modern borders, modern lakes and streams. Such data might be available (with the proper security clearance) for Afghanistan, but not for Normandy or Saxony as it was in 1945, much less for Flanders in 1916. Instead, folks like me who do such maps bring together the best maps and descriptions of the period, try to figure out how those relate to modern locations, and then sketch or trace features by hand in Illustrator or similar programs. To represent features like front lines, troops, salients, or skirmishes, we use symbols that we think will easily be understood by folks who look at such maps—the same way weather maps use invented symbols for cold and warm fronts, cyclones, etc.