Because older families are wealthier and immigrants who just came to the United States (people of color) is the cause of the gap. Not the race itself (Indian, Japanese etc..)
>The household median net worth was $247,500 for whites; $8 for US blacks (the lowest of all five cities); $12,000 for Caribbean blacks;
Note that Caribbean blacks (immigrants) have a much higher net worth than US blacks.
[0] https://www.bostonfed.org/publications/one-time-pubs/color-o...
[1] https://www.bostonglobe.com/metro/2017/12/11/that-was-typo-t...