Why did Sauron attack in the North as well?

[doug*platypus]

Minas Tirith was Sauron's main enemy, as stated several times in the book (Boromir, Denethor, Gandalf). However he had many enemies that he wished to deal with. Just because the "heaviest stroke" fell on Gondor doesn't mean that this was Sauron's sole focus. As has been stated he wanted nothing less than the complete domination of Middle-Earth.

The story is told from the perspective of people who were either at the Battle of Pelennor Fields or in the case of the two heroes, in Mordor. This means that we are biased into thinking that the fight for Minas Tirith was the only important battle of the War of the Ring. If Sauron had been more focussed on taking Lothlórien, then Gandalf would most probably have been there and the focus of the story would be quite different.

The fact that there were other battles going on in the north that we do not hear about underscores the fact that the entire world was under threat, not just "the world of men" as the movies are too quick to point out. Elves and Dwarves were under attack in their very homes. The conversation between Legolas and Gimli about their homelands being under attack is very important in terms of placing us in the context of what was happening at the time.

Also I don't think that sheer weight of numbers would have decided the Battle of the Pelennor Fields. Logistics have to come into play. Osgiliath was a choke point, and for Sauron to ferry every single one of his armies across would have taken an extremely long time. I think that Sauron committed as many troops as he could reasonably get to fight at Minas Tirith, as well as the Corsairs and the army that was set on the road to Rohan. Any further forces were better used elsewhere or kept in reserve.