Europeans first arrived at the new world seeking trade with India and china. They did this because the ottoman empire came to control the traditional trade routes. By the time the wealth of the new world was realized Europeans had become to powerful to challenge. The ottomans never had much interest as they were already closer and in control of the trade with the east, as well as controlling a sprawling rich empire with a constant threat or war in eastern Europe.

Mali collapsed before the new worlds wealth was discovered and the states that came after it lacked the technology, desire, and development required to colonize.

Al-mansur did state he wanted to start a Muslim colony but his attention was focused on taking the vast gold riches of the soghai (who inherited the Mali gold mines.) Before Pizarro took over the Incan empire the mines in western Africa accounted for roughly half of the old worlds precious metals. You can see why he wanted to first secure those resources before challenging the Europeans. Whim he relied on to counter the Ottomans.

I'm on mobile now so I really condensed things. Just ask away and I'll answer any further questions when I get home on my computer.