Melian's Change

[Maédhros]

After Thingol died, why was Melian changed. Why were her powers diminished?

But now Thingol lay dead, and his spirit had passed to the halls of Mandos; and with his death a change came also upon Melian. Thus it came to pass that her power was withdrawn in that time from the forests of Neldoreth and Region, and Esgalduin the enchanted river spoke with a different voice, and Doriath lay open to its enemies.

Was it the Power of Love? Was it Morgoth's power that had become greater? Was it a design of Manwë?
Why did she abandon her people?

Thereafter Melian spoke to none save to Mablung only, bidding him take heed to the Silmaril, and to send word speedily to Beren and Lúthien in Ossiriand; and she vanished out of Middle-earth, and passed to the land of the Valar beyond the western sea, to muse upon her sorrows in the gardens of Lórien, whence she came, and this tale speaks of her no more.