The more convenient Gulangyu ferry hauls a lot of freight during the day. Many locals depend on it for their livelihoods, carrying literally chickens and vegetables on their shoulders to the markets on both sides. IIRC, the locals' fare is only about USD$1 roundtrip. I surmise that tourists are required to use the other ferry to reduce the traffic bottleneck as well as extract more money from them.
I enjoyed Gulangyu immensely, but only because I wanted 2 full days of quietness and it was the perfect place for it. In the evening and early morning, you can really feel to have the island all to yourself -- and the cats. There are lots of them on the island.