Though it's viewed as a major turning point in Essosi and even Westerosi history, I haven't been able to find a reliable source that conclusively answers this question. Is there no historical consensus on the matter?
According to Maester Yandel, the sorcerers who maintained the Fourteen Flames were assassinated during several of the Valyrian dragonlords' scrambles for power.
With no one left to control the Fourteen Flames, earthquakes and volcanic eruptions eventually destroyed the Valyrian Freehold and transformed the peninsula into the landmass we have today.
Well, most scholars tend to agree that the Doom was caused by a series of volcanic eruptions, and earthquakes , which would explain why the area is so inhospitable