Because until pretty recent history that was what all countries did. It wasn’t considered bad by the morals of the time or at any point in history prior. Slavery has existed as long as humans have existed.
Countries got strong and took over over countries for their resources and power and created Empires. All across the span of history that’s what humans have done, conquered, enslaved and exploited those weaker than them. Assyrian’s, Egyptians,Greeks, Romans, Viking’s, Mongals, Chinese, Aztecs, Spanish, Ottoman, Russian, French, British……The British industrialised the process and were some of the more recent colonials so by nearness in time and scale of suffering they (we) get most of the current flack.
Whats out of pattern is the change in the last 100-150 years that has lead to the majority of humanity (not that I can speak for them all) thinking slavery is bad, colonialism is bad. (I’m not disagreeing)
A Britain enslaved by Rome in 50CE or whatever was probably not very happy to be a slave and have all these folks taking his land but he’s probably mad that HE’S a slave and HIS land is taken not that slavery exists, and given half a chance he’d likely enslave the Roman
What happens to make Britain decide slavery for everyone was bad and give up its Empire (under pressure) and for the predominant moral view to be slavers bad, empire bad? (and I don’t think it’s because the majority of historic brits were just amazingly good folks - clearly some of them were like the abolitionists) Because our current view are so out of kilter with the predominant morality throughout history.