First, the "narratological" answer: The "shadow host" is, as it were, the
narratological equivalent of a "one-trick pony" in a circus. The pony
comes out, does its trick; but it isn't brought out again because the audience
doesn't want to see the same trick performed again -- it wants something new.
That's one answer, concerning the craft of the author who wants to keep readers
entertained.
But here is another, which gets closer, perhaps, to how you
intended your question. The Dead inhabit the tunnels beneath the Ered
Nimrais -- unquietly. They have not been allowed to leave the Circles of
the World -- i.e. to receive Ilúvatar's special gift to Men, death and the
release which death brings. The reason for the denial of release from the
world is clear: while they lived, these now unquiet Dead had refused to fulfil
their oaths to Isildur. For three-thousand years they have suffered a
sort of un-life -- until such time as they fulfil their oaths to fight against
Sauron; if not to Isildur than to Isildur's heir. This is why the Dead
obey Aragorn, and no-one else. He can set a condition for the fulfilment
of their oaths and thereby allow them to die truly and to leave the Circles of
the World, to escape from the un-life of their wretched limbo. This does
not mean that the Dead are automata for Aragorn to command: the Dead too have a
right not to be taken advantage of. Aragorn is doing them a great favour
by alloting to them a fair task and thereafter holding their oaths
fulfilled. Furthermore, the Dead don't have to obey him -- they can
refuse to follow him as they once refused to follow Isildur. Precisely
because Aragorn gives them a fair deal do they follow him. He doesn't
take advantage of them and treat them as his slaves; he lets them fulfil their
oath and be released from the world forever.
People accept Aragorn as their leader because he only makes
fair demands, demands which people have a chance at fulfilling. Take the
example of the march to the Morannon, RotK, "The Black Gate Opens": When many
of the soldiers of Gondor and Rohan, despairing of this potential suicide
mission (the troops of Mordor will outnumber them ten-to-one and they all know
it), get nervous about going on, "Aragorn looked at them, and there was pity in
his eyes rather than wrath; for these were young men of Rohan...or husbandmen
from Lossarnach... they now walked like men in a hideous dream made
true... 'Go!' said Aragorn. 'But keep what honour you may and do
not run! And there is a task which you may attempt and so be not wholly
shamed. Take your way south-west till you come to Cair Andros, and, if
that is still held by enemies...then retake it...' Then some being shamed
by his mercy overcame their fear and went on, and the others took new hope,
hearing of a manful deed *within their measure that they could turn to*, and
departed."
Instead of sending them away as cowards and shaming them
forever, he assigns to them another task -- and lets them earn honour in this
way. He doesn't take advantage of them or force them to do something they
fear to do; he allots to them an honourable task *within their measure*.
That is leadership. And Aragorn treats the Dead in the same way.
That is why he doesn't take advantage of them by setting impossible demands or
by treating them as his slaves. He gives them a fair deal, and they take
it.
__________________________________
All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.
From the ashes a fire shall be woken,
A light from the shadows shall spring;
Renewed shall be blade that was broken,
The crownless again shall be king.
