What's this?!

[Kuruharan]

Maybe it's something that depends upon the situation. A large and epic scale war such as the one's the Gondorians were facing here, it's much easier and time efficient to have one leader in charge making the decisions...and not several lords doing their own things with their armies.

Are we supposing that the kings had greater control over the Lords?

As I said above, societies where power (in this case basically reduced to who has ultimate control over the military force) is broadly spread at the top do not generally go forth and conquer empires (at the moment I can't think of a single historical instance where such a society did so, but am open to examples).