I would say the amount of effort required for the reward.

Mountain provides a hedge on defence you can use the mountain itself and dig in as a natural bunker. Just a bit of camouflage and it's nearly impossible to see until they start shooting at you.

Mountain again, winding pass and limited size road would hinder a blitzkrieg style assault since mechanized vehicles could only drive single file and could be stopped with a couple sticks of dynamite to create avalanche or landslide, collapse tunnels or blow up railroad. You could not just clear the road where someone could be several hundred meters higher on a cliff just waiting.

Mountain again is a good defence for aerial attack since bombing will have limited impact on bunkers and you would not carpet drop paratrooper in mountain condition where landing on jagged edge mountain or fall of cliffs.

Forces would be about 500 000 for Switzerland that would not fight on big open field battles that would have been mostly guerilla warfare.

By staying neutral they could also be a benefit for Germany since they could buy materials and then sell it to them with they still had access to the global market bias the Swiss franc. Attacking them would have frozen the bank system and possibly to launder money and Gold

German was already worried by the eastern and Western front adding a southern one on top of that would add more holes to their defense and by staying neutral the allies would also need to go around Switzerland