That's not entirely true, Gaea for instance appears more than once in the myths, even after the castration of Ouranos (Note that it is not mentioned that Ouranos is killed, just castrated and then consequently bound). The first instance is when Kronos starts eating his own children, Rhea goes and weeps to her mother (Gaea) and asks her for advice. Gaea then suggests to her the plan to replace her youngest son(Zeus) by a stone, and smuggle him to her so that she could take care of him. She then ensured that Kronos could not hear young Zeus' wailing by having Naiads disguise the sound by smashing rocks together.
She later helps in training young Zeus to overcome his father.
Hesiod, Theogony 462 ff (trans. Evelyn-White) (Greek epic C8th or C7th B.C.) :
"[Kronos (Cronus)] learned from Gaia (Gaea, Earth) and starry Ouranos (Uranus, Heaven) that he was destined to be overcome by his own son, strong though he was, through the contriving of great Zeus.Therefore he kept no blind outlook, but watched and swallowed down his children : and unceasing grief seized Rhea. But when she was about to bear Zeus, the father of gods and men, then she besought her own dear parents, Gaia and starry Ouranos, to devise some plan with her that the birth of her dear child might be concealed... Him did vast Gaia (Earth) receive from Rhea in wide Krete to nourish and to bring up...
She appears once more (that I am aware of) during the war of the Giants, Gaea is angered by the actions of the gods, especially that of Zeus so she conspires with Tartarus and gives birth to her second set of children, the Giants. Each of whom is destined to fight against one of their counterparts in the Greek gods.
Pseudo-Hyginus, Preface (trans. Grant) (Roman mythographer C2nd A.D.) :
"From Terra (Earth) [Gaia] and Tartarus [were born], Gigantes (Giants) : Enceladus, Coeus, elentes, mophius, Astreaus, Pelorus, Pallas, Emphytus, Rhoecus, ienios, Agrius, alemone, Ephialtes, Eurytus, effracorydon, Theomises, Theodamas, Otus, Typhon, Polybotes, menephriarus, abesus, colophonus, Iapetus."
Ovid, Metamorphoses 1. 156 ff (trans. Melville) (Roman epic C1st B.C. to C1st A.D.) :"Gigantes (Giants), it's said, to win the gods' domain, mountain on mountain reared and reached the stars."
But it is also true that they don't appear often in the works of the more recent Greek author, this can be attributed to the fact that most of these authors focused on the age of Heroes(wherein the Titans and the Elder Gods did not play an important role.), the Gods were merely the procreators in this age and in some cases the barrier/nemesis that the Hero had to overcome in his quest for Glory (for example Heracles and Hera).
The second part of the question has more to do with Anthropology(if I'm interpreting your question correctly), It is highly likely that the Elder Gods were Deities of a tribe that was either defeated or assimilated into another tribe. It could also be an indication of the changing belief systems of the tribe, a shift in focus as such from more general beliefs such as Sky and Earth to more specific domains that they believed would help in their various ventures, such as Agriculture(Demeter), wishing for fair weather at Sea(Poseidon), etc.
A very good resource for this would be the Greek Myths by Robert Graves, he has very concisely built a case for the development of the various myths, and a corresponding event that may have occurred.