Why did timbuktu decline in influence?

The city began to decline after explorers and slavers from Portugal and then other European countries landed in West Africa, providing an alternative to the slave market of Timbuktu and the trade route through the world's largest desert. The decline was hastened when it was invaded by Morisco mercenaries armed with European-style guns in the service of the Moroccan sultan in 1591.

Timbuktu was at one time the center of the world for learning.

the enslaved people onboard suffered from illness with some frequency

there were abundant sources of fuel nearby