"But One and not the Greatest"

[NightKnight]

I don't think Sauron saw the need to send out all of his armies against Gondor. As has been said before, the army WK commanded outnumbered the gondorian one greatly, and the fear he would put into men was an extra bonus. Apart from that he had the corsairs to distract the people from the coast and he had sent an army over Cair Andros to ambush the rohirrim (which failed because of the druedain).
I don't think he thought WK could be defeated, that Aragorn would summon a long lost host, that the winds would turn, giving his enemies hope and speed, and that the rohirrim would be able to escape from the Cair Andros-army. So it was pretty much a bunch of lucky things that made Sauron lose that attack.

So great an army had never issued from that vale since the days of Isildur's might; no host so fell and strong in arms had yet assailed the Fords of Anduin; and yet it was but one and not the greatest of the hosts that Mordor now sent forth.

He did attack on several points at about the same time. Three attacks on Lorien, one on Mirkwood, one on Dale/Erebor, one on Rohan, and he still had a huge army left to surround the attackers at Morannon. I think that one was the largest one, to protect himself from attack.