If jesus gave a new covenant...?

why did christians still believe slavery was ok until only about 100 years ago? why did they believe murdering the non-believers, i.e. native americans, was ok a few hundred years ago? why in some states was interracial marriage banned until less than ten years ago? christianity had over 1500 years to spread from the time of jesus till the oldest events i mentioned. and these weren't unpopular practices among christians either. they were very common. shouldn't the new covenant have allowed for blacks to remain free? shouldn't the new covenant have allowed for the native americans to practice their own religions? shouldn't the new covenant have allowed for interracial marriages?