In that case it is all about frequency. More trains means less average waiting time. Because the trip is precalculated and people can leave there home at any time, they do not know if they will catch the fast train or have to wait for it for a long time.
If there is one fast train on a trip which takes 4 minutes to go (8 minute roundtrip), than people expect 4 minutes of average waiting time, so wait 4m + travel 4m = 8min.
Now say you have two slow trains for the same route and it takes 5 minutes (10 minutes roundtrip), maybe because of an additional stop. But people expect a waiting time of average 2,5 minutes, because there a two trains on the route. So in that case, wait 2,5m + travel 5m = 7,5min. Therefore it is preferred.
And with every train you put on the “slow line” it will get even more attractive, as waiting time drops.