Because this depends on a particular verb, in substance, what would be a proper subject (the perfomer of the action) for a particular verb.

Examine these:

Я люблю её (I love her) - Я is the subject that always takes nominative case, люблю is the predicate and a verb, её is the object the predicate takes. An object never takes nominative case but some other (usually accusative)

Мне нравится она - Мне is the object the predicate takes, нравится is the predicate and a verb, она is the subject, the performer of the action.

So the last example sentence could be literally translated as -- She is liked by me.
Of course, the proper translation is - I like her.

Another example:

Мне нужен стул - Мне is the object, нужен is the predicate, but not a verb, стул is the subject.

This one is translated as - I need a chair.