Someone (Everton, Peel or the Council) would have to come up with the money. The new landing stage and gangway at Woodside oost £9m, so add on another couple of million for the quayside works for it to connect to. If you don't want to impact the existing ferry operations you would need a second ferry - the new Mersey Ferry is going to cost at least £20m (you can't just use any old boat, it has to be capable of handling the tidal currents and swells found in the Mersey, and the football season runs through the seasons when you get the worst weather), which outside of matchdays would probably be laid up as the existing ferry is enough to handle the commuter and tourist trade.
The current landing stage at the Pier Head only has a single berth as well, as Merseytravel had reduced the service to a single ferry operating a triangle route by the time the 1975 landing stage sank for the last time in 2006. You would also need to pay the crew overtime on weekday matches, as ferry operations finish around 7pm.
That's before you get onto any dredging that's required. The area from Sandon Dock to Salisbury Dock has a depth at low tide of 8-8.5 feet (the same as the draught of the current ferry) from 5m out from the dock wall to 250m out into the main channel of the river. There is also a 2m diameter pipeline on the river bed that runs 250m out into the main channel of the river immediately to the north (from the former Sandon Dock gates), carrying the outfall from Merseyside's biggest sewage treatment works next door to the stadium (Peel were presumably very happy they found a sucker to take on the Bramley-Moore site, as it was going to be a difficult sell for leisure or residential developments). That would be a hazard for any dredging operations and ferry nevigation at low tides.
You can see why they have gone for the cheap option of running buses from Liverpool One.