The Point of the Unions

[daeron]

Arwen had to turn mortal becaue the last ships which can take her back were already leaving ME. The Valar were setting down the last dates for return. Arwen cannot stay till Aragorn's death and later take a ship. Remember the roads were bent forever.
Luthien turned mortal only in her second reincarnation and that too because she wanted Beren to live a second life. She would have been reincarnated anyway, but she could not bear to live without Beren. She died of grief the first time. She turned mortal voluntarily to save herself pain.
Idril on the other hand may have agreed to live even after Tuor dies. As to why Tuor should turn immortal, I know not the answer. Maybe they stopped giving out "Elven tokens" from second age onwards. Also, would Elessar have been able to leave everything he had fought for and leave ME. I think no. What would have happened to ME if he had left. The Valar would also have to take that into consideration. Afterall, love stories aren't their only worries.