The reason is because Document A is the primary source written to the king.
Step-by-step explanation:
Document A is said to be reliable source which helped to understand what had actually happened when Moctezuma had met Cortes. This was considered to be a reliable source because this was the primary source which was written to the king Charles V.
Because it was a primary source, it was obviously a reliable source. This document was a letter written to the king which described the meeting of the two.