That's because from court point of view, he already admitted to bribery which is chargeable in court, but the receiver hasn't admitted receiving it, now they prosecution team need paper evidence to show that receiver received the money, without evidence it's just a waste of court time. And evidence with just that guy saying he give, is not a strong enough evidence

Please note, I've no idea what is involved in this case, videos, documents, I'm just saying one of a possible scenarios