Logistically, it would be a nightmare to go through Switzerland. Not only is it mountainous, compared to the very flat Belgium, but there were relatively few rail lines running from Germany, through Switzerland and into France, compared to Belgium (necessary considering rapid mobilization and deployment was critical at the outset of hostilities). Also, just look at a map and look at the relative location of Paris to Belgium and Switzerland. This is important to early 20th century thought on warfare.

Both France and Germany had plans revolving around a single battle, knockout style of attack. The Germans, guided by the Schlieffen Plan which had been devised and discussed/debated/modified pretty much since the Franco-Prussian War, focused on a buildup of forces on their right (north or near belgium/the channel) flank. The French, guided by Joffre's Plan XVII, was the opposite. They also built up forces on their right (south or Alsace-Lorraine) flank. Both the French and Germans wanted to thrust into their opponent's territory from the right flank. They each welcomed the other side building up forces at the opposite flank. They were each relying on their concentrated forces breaking through and forcing a decisive battle on their strong flank, and hoping that their weak flank would simply hold up the opposing forces long enough for the attacks to succeed.

This is speculation on my part, but a German route through Switzerland would expose the entire supply line of their forces to the main concentration of French forces. It would also take the bulk of their forces into the heart of the continent, leaving them dangerously exposed to a landing by the BEF either on German soil, or a thrust through the low countries. By going through Belgium, they were going through the weakest point on the French line, and if successful they could put immediate pressure on Paris, possibly knocking France out of the war so that they could transfer troops from the West to the East (which they did, thinking the Battle of the Frontiers was decisive). Going through Switzerland would have delayed all of these things and taxed their supply lines severely.