The correct order of the events is as follows:
1. The Portuguese establish sugar plantations on islands off the coast of West Africa;
2. Portuguese laborers are unwilling to leave their homeland;
3. The Portuguese bring in slaves to work on their plantations;
4. Other European countries also start purchasing enslaved Africans;
The Portuguese didn't really investigated the situation about the labor force before they make sugar plantations, so they set them up, and it turned out that the Portuguese people are not willing to come and work on them, so they were left with plantations without laborers. Since they didn't wanted this investment to be for nothing, they started buying African slaves from some of the stronger tribes that were keeping slaves. They used them as labor force afterwards, and saw the long term benefit of it, so started to purchase more and more slaves. After the word spread out, and also after the other European countries started to have colonies, they too started to purchase African slaves, thus making it a huge business for both, them and the stronger African tribes that were selling the slaves to them.