Because the rest of the classes do not have a specific set of rules for the class. It is really easy to justify almost any discussion based off of your own alignment.
Barbarian: Can not be lawful. So as long as I make sure to break the law every once and a while or at least think about not doing something just because it is a law I'd be fine. As a raging hulk, that sounds pretty easy to do.
Monk: Just make sure to reinforce the fact that your character meditates each day and that should counter most Chaotic things that you do. That and don't go out of your way to break laws.
Druids: Now it gets a little easier to fall. Druids need to not piss-off their deity and love nature. So choose a deity that fits your play style and only play a druid if you like respecting nature. Easy. Oh! And avoid metal armor .... once again, easy.
Clerics: This can be a little harder as people like to choose deities that have good domains. But, once again, there are no hard and fast rules for grossly violating the code of conduct. So if you are worshiping Desna, don't get slaves. If you are worshiping Gorum, don't try to stop nations from fighting with each other.
Paladin: Now this is easy to fall from. All others have been vague about what it takes. Paladins, on the other hand, " A paladin who ceases to be lawful good, who willfully commits an evil act, or who violates the code of conduct loses all paladin spells and class features. " This is hard and fast. The first Evil act you do and you lose everything. The Code is strict "a paladin's code requires that she respect legitimate authority, act with honor (not lying, not cheating, not using poison, and so forth), help those in need (provided they do not use the help for evil or chaotic ends), and punish those who harm or threaten innocents." Meaning there are solid lines in the sand that they can not cross.
For other classes to fall, they need to not only be constantly breaking their code enough to change alignment, but they can also justify most actions to fit in with their character. For Paladins, there is no second chance, there is not justifying. There is only the first mistake and then the fall.