I think the problem is twofold.
1. The ferry companies have to make separate arrangements for foot passengers to board.
2. There is probably a limit on the total number of passengers carried and they'd rather carry someone with a car who they can charge more for.
There is also the challenge and cost of providing UK▸ Boarder Force, French Police Aux Frontières and Customs checks for relatively small numbers of foot passengers
My understanding is that you can still travel on the ferries without private transport using Eurolines and perhaps other coaches who say that some services use the tunnel and some use ferries. I'm not sure in what proportion or what the future holds there.
Perhaps the nub of the problem is that (as I understand it) International travel from (and to) the UK is private operated, with no contract / subsidy provided to operators which require them / pay them to take any particular type of traffic and, yes, foot traffic has reduced to the extent it is not commercially viable. P and O Ferries have not gained themselves a positive reputation for looking at social needs.
To add, yes, international travel from and to the UK has become a much more complex with border controls and security much more expensive to provide in recent years.
