The studio probably figured it wouldn't spark any controversy since Shirley Temple was 6 years old at the time.The first interracial hand-holding actually appears to be in The Littlest Colonel between Shirley Temple & Bill Robinson from 1935 (another 8 years earlier).
There was a Japanese-American actor named Sessue Hayakawa who romanced Asian and Caucasian women in silent films. So, maybe he was doing the interracial thing in movies first.
Based on the racial standards (and racial hard-on)of America and how America views race (i.e. only discussing it or bringing attention to it when something involves black people opposite non-black people) it always bothers me when people gloss over the 'other' interracial kisses Kirk does in the series with non-white women. For example, the smooches with France Nuyen and Barbara Luna.