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Why some ferries (like Liverpool to Dublin) only allow passengers with a vehicle?

In a trip some time ago I was in London and my next stop was supposed to be Dublin. A friend of mine had just moved to Liverpool and I discovered there was a ferry from Liverpool to Dublin. I thought I could change my plans, go visit him and take this ferry. I tried to buy a ticket online, but to my surprise I could only embark with a vehicle! Passengers without one could not go aboard! As I had no vehicle, my choice was to take a ferry from Holyhead (Wales) to Dublin or fly directly from London (which I did).

Does someone know the reason why only passengers with vehicles are allowed on some ferries? Is it common? I've only traveled once in an international ferry, so I don't have much experience ferrying around.