Because the two are so highly correlated that having both would give us almost no extra information. One goal of the survey should be to maximize the useful-info-extracted / time-spent-on-it ratio, hence also the avoidance of write-ins for many questions (which make people spend more time on the survey, to get results that are less exploitable) (a write-in for gender works because people are less likely to write a manifesto for that than for politics).