The presumption is you are ditching with no engine power, therefore no normal electric power, and therefore no hydraulic power generation as you drop below flight speed. You don't want to waste the remaining stored hydraulic energy on unneeded brake actuation when you may need it for flight control movement after the first bounce or two off the wave tops, or to try to keep the gear doors shut as stated by BBK above. I suspect the gear doors get ripped off pretty early in the process, though, in an open ocean situation with waves.