The information is from older TEPCO press releases (they are publishing information a few times a day). The problem is that while the switchgear and substation equipment for 5 and 6 remained somewhat intact, the electrical gear for units 1 through 4 did not.
Also, one backup generator was inadequate to maintain both 5 and 6 along with their spent fuel ponds at an adequately cool level. They would have endangered those two units to provide an inadequate supply to the other 4 generators. Essentially, if they could have made the hookup, they'd probably have had the roofs blowing off 2 more units and would have 6 scuttled nuclear reactors instead of (probably) 4.
They needed to wait for off-site power to provide adequate supply to run pumps and cooling equipment on 1-4 (some of which is now damaged due to the explosions, earthquake, and tsunami). And even with that hooked up, they still need to adequately test each piece of equipment and circuit to be relatively sure that the are operational and are not faulted (short-circuited).