Because there is nothing obtainable in emptiness, including the Five Skandhas, Six Internal Bases (Six Roots), Six External Bases (Six Dust), Six Consciousness Eighteen Fields, Four Noble Truths, Six Paramita and even the attainment, and because Bodhisattvas abide by the Prajna-paramita, they have no attachment and hindrance in their minds. If there is one Dharma not yet empty. The mind is still attached and hindered.
If there is something obtainable, Bodhissattvas will not certify the Prajna-paramita. The Prajna-paramita is the wisdom to understand the reality character of nature and all Dharma, i.e. non-emptiness coupled with non-existence. Or, if there is no attachment to both emptiness and existence, then you are unimpeded and pure in your mind.