It’s a painful irony: congregations in mainline churches—which have long made racial reconciliation one of their highest priorities—are no more racially integrated than other churches, and in fact tend to be somewhat less integrated than independent and theologically conservative churches (see John Dart’s "Hues in the pews").
One should refrain from too much moralizing over these statistics. As the researchers point out, demographic and historic reasons help explain the mainline situation. And few congregations in any Christian tradition can claim to exhibit a rainbow of humanity. The reality of congregational life rarely matches the diversity featured in denominational literature.