You could simply use the Fall of Netheril itself as reason enough, since during that time even magic itself stopped working. Personally, I would find that a convincing reason for people to believe it to be a magical dead zone.
However, any disaster, mundane or otherwise could keep people from attempting casting in the area/region. Fear can be a powerful thing.
As an alternate to Netheril you could use something like the Tomb of the Astronomer as the center of some failed (or believed failed) magical experiment, although you'd need to change the region for your own setting.
Some of Faerûn's most appalling battles occurred during the Era of Skyfire. Battle accounts ink crumbling tomes with names such as the Blood March, the Fall of Agis, and the Battle of Ruin.
In Calimshan, tales speak of a lost reliquary contemporary with the famous djinn Calim. This ancient place, called the Tomb of the Astronomer, remains lost in the midst of the Calim Desert, and some say it contains an ancient secret.
Finally, why would any wizard be out there in the first place? Maybe it is thought of as a dead zone simply because so few casters are interested in it.
Sundry other and/or related reasons:
- The area is simply too forbidding for most mages to deal with.
- Those who do cast there keep it secret because they do not want competition for the lucrative services they provide to caravans along the Black Road.
- Those who know that magic works there want no competition from others seeking the secrets of Netheril. (Or whatever secrets you choose to bury there.)
- Those who know that magic works there live in fear of those who fear a repeat of whichever great disaster occurred (or believed to have occurred) above.
- Those who fear a disaster (or a repeat of an earlier one) spread rumors to discourage anyone else experimenting in the area.
- The locals are superstitious against magic users, for either real or imagined, possibly historical, reasons. May or may not be related to the groups above.