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).
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).