The reason for such construction may probably have been caused by the inevitable thin layer of earth covering the section between Tofukuji Station and Shichijo Station going underground (from aboveground), which made the construction of a two-level underground structure impossible.
このような構造になった理由は、おそらく東福寺駅-七条駅間で地上線から地下線に切り替わるために土かぶりを浅くせざるを得ず、地下2層分の施設が造れないためだと思われる。