Comparison to English might help here. (English is often misleading for Latin, but here it can at least illustrate the same phenomenon.) English distinguishes between nominative and accusative for personal pronouns, for example "he" vs. "him". Therefore the best analogue is found with pronouns instead of nouns.

Consider the sentence: Ille amandus est. Why is it not illum? Compare this with English. We say "he is to be loved", not "him is to be loved". The passive meaning comes from "loved" being a passive participle. The pronoun "he" is the subject of "is" and therefore in nominative.

This works the same way in Latin: the gerundive (or a perfect participle) is passive in meaning.

It is also worth noting that there is the object-free option legendum est, "one has to read". An object can't be added to this kind of construction; instead, you get nominatives like in your example. This relation is explored in this question.

Comparison to English might help here. (English is often misleading for Latin, but here it can at least illustrate the same phenomenon.) English distinguishes between nominative and accusative for personal pronouns, for example "he" vs. "him". Therefore the best analogue is found with pronouns instead of nouns.

Consider the sentence: Ille amandus est. Why is it not illum? Compare this with English. We say "he is to be loved", not "him is to be loved". The passive meaning comes from "loved" being a passive participle. The pronoun "he" is the subject of "is" and therefore in nominative.

This works the same way in Latin: the gerundive (or a perfect participle) is passive in meaning.

It is also worth noting that there is the object-free option legendum est, "one has to read". An object can't be added to this kind of construction in classical Latin; instead, you get nominatives like in your example. This relation is explored in this question.