It was not due to the alps but of there economic importance. alps would have only been minor problem.
"The reason Germany spared its tiny neighbor to the south was because Switzerland proved much more useful as an independent state than as a satellite. The Swiss made many useful weapon components (aluminium for the Luftwaffe, spark plugs for jeeps taken from the Russians, timing devices for bombs, among other things), and thus their factories were not bombed every night. The Swiss National bank bought gold from the Reichsbank, the Reichsbank was given Swiss francs in exchange, and used them to buy cobalt, nickel and tungsten from the other “neutral” countries. The Turks, Portuguese, Spanish and Swedish, who were all under heavy pressure from the Allies not to accept direct gold payment from the Reichsbank, then exchanged the Swiss francs for gold."
much more info in my source.