Míriel's death

[Maédhros]

If the elves were inmortal, why then did Miriel had to die.

But in the bearing of her son Míriel was consumed in spirit and body; and after his birth she yearned for release from the labours of living.

Why couldn't she find healing in Aman.

'Surely there is healing in Aman? Here all weariness can find rest.' But when Míriel languished still, Finwë sought the counsel of Manwë, and Manwë delivered her to the care of Irmo in Lórien. At their parting (for a little while as he thought) Finwë was sad, for it seemed an unhappy chance that the mother should depart and miss the beginning at least of the childhood days of her son.
She went then to the gardens of Lórien and lay down to sleep; but though she seemed to sleep, her spirit indeed departed from her body, and passed in silence to the halls of Mandos. The maidens of Estë tended the body of Míriel, and it remained unwithered; but she did not return.

Was this made because of the importance of Feanor in the Silmarillion or it was a fault or singularity regarding the supposedly inmortality of elves.