Because it would have been much much harder. Switzerland is surrounded by the Alps. A mountainous region that very much favours the defenders and is a huge strain on logistics for the attacker. Getting through Switzerland would have been a task in itself instead of a stepping stone to France. The Swiss also built a series of fortifications and could swiftly mobilise the Landswehr/reserves. Its policy of armed neutrality was to discourage such attempts by any warring party and they were prepared to fight. All of this would give France ample time to reinforce the swiss and the border between them.