The nature of the Flower Forests is unchanging and eternal. They exist as reservoirs of the Light, and as such the Elven Mages (Lightborn) of Vieliessar's time were taught to draw on them for the power to work their spells (as opposed to the extremely skanky practices they got up to before Mosirinde Peacemaker reformed them). The Lightborn Elves who bond with dragons in Vieliessar's time basically don't need the Flower Forests any longer and have an absolutely infinite resource of Power to draw upon.

Over time, there stopped being Lightborn Mages (since among other things they were too vulnerable to the Endarkened), and the only kind of Elven Mages there were, were Dragonbond Mages. When the Dragons turned to bonding with Humans (once there were humans), the Elves lost all magic but their connection to the "small magics".