For context, I'll repeat the quote here: "one survey reveals that about 90 percent of the temple marriages hold fast. Because of this, we recommend that people marry those who are of the same racial background"
Good question, I have no idea, since none of those have had any bearing on temple entry in 1976, save religion and race,. All I have to go on is what SWK said, and really, it's that 'Because of this' qualifier that links 'race of spouse' to temple entry. SWK also says that, "some of these are not necessary, but preferred".
It appears to me that, in 1976, race and religion were the only two things necessary for a temple marriage. Which leads to more questions - if the other factors are 'preferred', by whom are they preferred? Do other modern prophets and apostles preach that one should make sure that one's future spouse have similar educational, economic, social and racial backgrounds?
In my experience, I've only heard leaders counsel us to marry someone who is, at most, worthy to enter the temple, and at least, shares the same moral values that we do. I've never heard it said, other than in this one place, that spousal choice should be made based on other preferred factors.
Well, I've hesitated to put too much spin on what SWK said, but since others have, maybe I, too, have been granted some license. Perhaps SWK was sensitive to the social climate of the time, and didn't want to draw attention to black/white intermarriages, and so he just covered all of them with one phrase. Of course, I can't know this anymore than anyone else can know if their own interpretation is correct.