because they wanted more good's and land if i remember correctly............... :) ;)
The dominant reason for Morocco's invasion of the Songhai Empire was to dominate the Trans-Saharan Salt-for-Gold Trade. The Songhai Empire was wealthy with gold, and this attracted the Moroccans. While the Songhai military was very skilled, it was no modernized, allowing Moroccan armies using firearms to overrun the empire in 1591.
It is worth noting that there was never a motive of conversion since both Morocco and the Songhai Empire were Sunni Islamic States and that the Songhai had been largely peaceful towards the Moroccans prior to the invasion.