[rcxdude]

The frequency of the grid is a strong indicator of the balance between supply and demand. If it has drifted off by even a small amount the system is close to a runaway collapse (modern switch-mode power supplies are actually quite bad from this point of view: when the voltage drops their effective power draw actually increases, in comparison to a lightbulb or traditional motor). And when the system collapses in an uncontrolled way it can quite easily cause severe damage to generating equipment which will take it offline for a long time. So there's a lot of stuff which goes into cutting out load and/or generators before bad stuff happens, and it tends to err on the side of caution, because a 1 hour blackout is far better than a significant amount of generation capacity being offline for months.