Colonists were highly dependent upon slave labour, and were hugely resistant to anything which threatened their livelihood. Many plantation owners assumed that the Amelioration Act was a step on the road to total abolition.
The provisions of the Act which required food and education for slaves represented a cost to slave owners that they were unwilling to undertake.
Although the Act purported to give slaves certain rights, very few slaves were aware of these rights, and even if they were, they were in no meaningful position to enforce them. Many of the judicial authorities were plantation owners themselves, and had no enthusiasm to pursue those who broke the law.
It is important to remember that the Amelioration Act was not wholly beneficial to slaves; it also included restrictions on them; including, curiously, prohibiting them from engaging in a Christian marriage.