I want to enquire about the concept of slavery in islam? Although islam has made betterment for slaves but many non muslims quote that if islam is a religion of freedom, then how come it has not abolished slavery!

Islam practically abolished slavery. It wasn’t effective to abolish it overnight because many slaves would have perished. They did not have the means to support themselves. Islam encouraged Muslims to free slaves and sometimes compelled them to do so (like in paying a Kaffara). That’s why you see slavery ended in many Muslim societies.