В Dictionary of Basic Japanese Grammar (Makino, Tsutsui) написано:
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In some situations, if the verb in Vmasu tai is a transitive verb, the direct object can be marked either by ga or by o. In general, the choice between ga and о seems to depend on the degree of desire. That is, when the desire to do something is high, ga is preferred; when it is low, о is used.
Under the following conditions, ga cannot be used even if the degree of desire is high.
(A) When a long element intervenes between the direct object and the verb.
(B) When the main verb is in the passive form.
(C) When the preceding noun is not the direct object.