Very easy:
One is founded on libertarian ideals. The other one is founded on state-mandated pluricultural sectarianism.
In switzerland, plurality is represented via multiple presidents and they can represent any canton.
In lebanon, plurality is state-mandated: must have a president that is of X religion. A prime minister of Y religion. This is "representative".
The swiss has very strong traditions on union out of necessity (to face external threat).
Lebanon has no tradition of unity, it is a state unified effectively out of fear (and a bastard child of french palestinian mandate).
The swiss culture of unity is long: they worked 800+ years to establish their union and it was carefully balanced over many iterations of conflict, even internal ones.
Lebanese culture of unity is very weak. They basically were friendly with all neighbors seeking to influence internal policy. For example, all public works depend on a sponsor (saudi, qatar etc), to fund the project. Money buys influence. Imagine if FL depended on Venezuelan treasure to build roads. Very weak internal unity.
Lebanon was only the switzerland of the middle east because it was the most free of countries in the region. But the regional bar is very low. As it turns out, that only gets you so far.
And time ran out for lebanon... a while back:
When? I think Black september sealed the fate of lebanon. They had no internal unity to reject the palestinian refugees like Jordan did (via authoritarian methods). That permanently and irrevocably set them on the path they are today.
In summary, calling lebanon the switzerland of the middle east is a romantic idea for the lebanese that lived thru a somewhat prosperous but short time period of 15 or so years. Nothing suggested it could be long lasting. It was always an empty vessel. A student that cheats on every exam, will eventually get caught.
The rest as they say, is history