The demand may not be there because some people would prefer to use the subway (F,G - Smith-9th St and/or 4th-9th St) in that general area. As a conductor, telling people to take a bus instead of a subway...people look at me if I'm insulting them. So therefore the demand may not be there.
However, about a decade ago...maybe a bit longer, there was a proposal to extend the former B71 AND B77 to Lower Manhattan (most likely Whitehall St-South Ferry to connect with the 1, R and W along with other bus routes) via the Brooklyn-Battery Tunnel. This was when Whitehall St-South Ferry terminal was going through a makeover in anticipation of the M15 +SELECT BUS SERVICE+ ending at a NEW terminal IIFC....and the M15 local staying put.