His thoughts are still peaceful, his emotions are relaxed. He can supplicate Hashem with a relaxed, peaceful frame of mind. He can have the proper kavanah, concentration and intention. Likewise, at the end of the day, regardless of the day's trials and frustrations, business is now complete. He can relax, because nothing is going to change.
Tomorrow is another day.
Tonight he can pray to Hashem relaxed and at peace. Thus, Shacharis and Arvis are both tefillos that are prayed at a time when one's frame of mind is attuned to prayer. Tefillas Minchah is different. Because Minchah is recited in the middle of the afternoon, at a time when one must invariably take time out from his hectic schedule, it is difficult to maintain the proper concentration. One must block his thoughts regarding a favorable business deal, rid one's mind of his daily business affairs, in order to daven with total kavanah.