The Swiss have many large mountains and so a campaign into Switzerland would be very hard. Paris is also much farther away if you invade through Switzerland and the German Schlieffen Plan called for a quick, decisive war against France which would be much easier to achieve traveling along a shorter route. To supply the massive German army, railways were needed and these were much more prevalent in Belgium then Switzerland. If you look at a map of the railways in Europe in 1910, you can see the number of railways going through Belgium from Germany into France is much larger than the number of railways going through Switzerland (https://etc.usf.edu/maps/pages/1700/1799/1799.htm).

TL;DR Belgium provided a faster route, had better infrastructure, and has fewer mountains than Switzerland- all of which were crucial to the German plan of a quick campaign to knock out France.

In addition to /u/zinkelburger's excellent points, Belgium was also a neutral country in 1914.

When it seceded from the Netherlands in 1836, it was widely feared that it would fall into the French orbit and so the Treaty of London that established Belgian independence included guarantees of Belgian Neutrality by the UK, France and Prussia, whose obligation was inherited by Imperial Germany. All three of those nations promised to defend Belgium if attacked, but also to fight against Belgium if she was the attacker.

However, British foreign policy since Napoleon (based on the principle of maintaining the Balance of Power in Europe and supremacy on the high seas) had persuaded many in both France and Germany that Britain was essentially pragmatic and self-serving. As a result, quite a few people in decision-making roles in Germany genuinely didn't believe Britain would go to war over Belgian Neutrality - Chancellor Bethmann-Hollweg reportedly called it a "scrap of paper".

Britain hadn't even really committed itself to an alliance with France: while various "understandings" had been arrived at - not least that the Royal Navy would guard the English Channel allowing the French fleet to deploy in the Mediterranean against Germany's then-ally, Italy - the British government was scrupulous about never actually putting it in writing, or promising anything definite.