Why was the slave trade allowed to continue for so long?

It was difficult for the slaves to reproduce so they had to get and trade slaves.  So finding and trading slaves was somewhat easier.  (This by my knowledge is the correct answer, though if it is not I apologize)

the fact that could also be added to column b is that during world war ii, specifically between the years 1941 and 1945, japan expanded its territory towards the indonesian territory in what was known later as part of the "pacific war". in that time, japan could conquer the philipines, singapore,   borneo, central java, malang, cebu, sumatra and various other islands of the area, therefore expanding its territory for a brief period of time.