Well the 'official reason' as stated by the Moroccan Embassy in the US is due to the amount of American shipping that stopped off at Tangiers. The question really is why were the Moroccans in such a rush to attract relations with these sailors.

Around that time American ships were having a lot of problems with Barbary pirates, which were made up of crews from Morocco, Algiers, Tunis and Tripoli, as they had lost the rights to passage they would have had as British ships, as the British crown paid tributes. The Moroccans saw the US as potential new customers, so to speak, and were happy to forge relations with them leading to the 1787 treaty. The other states, who had been far more aggressive on the seas, had to wait another decade before getting their tribute and trade rights.