The only way that it seems even remotely reasonable for High Elves to willingly turn to the Void, is if there are as yet unseen benefits to doing so (i.e. perhaps the Void, similarly to the Light, more thoroughly satiates the pangs of their magical addiction).
But even then, why not just join the Church of the Holy Light? This way they avoid most/all of the physiological deformities associated with becoming a Void Elf.