The crux of the matter is that “prince” and “princess” aren’t real titles of nobility. A real title goes with an “of Someplace” after it. If you’re the Duke of Something then your wife is also Duchess of Something. “Prince” and “princess,” by contrast, specifically denote that you’re the direct descendent of the Queen. Thus Prince Charles is a “prince” as are his sons, and his brother Prince Andrew and his daughters Princess Beatrix and Princess Eugenie. But Kate’s not descended from The Queen so she’s no princess.