NOTAMS contain information about potential hazards and other mission-critical information that is short-lived (so not worth printing on charts) or otherwise breaking news kind of stuff.

Without that information, pilots will be unaware of dangers that may be involved in their flight.

Not everyone cares, per se, which is why VFR traffic was exempt from the stop, and GA (which operates under looser regulations) were among the first to be exempted.

But airlines are heavily regulated and aren't allowed to depart without reviewing the NOTAMS relevant to their flight path and if the NOTAM system is offline they're unable to meet that regulatory requirement.

NOTAMS contain information about potential hazards and other mission-critical information that is short-lived (so not worth printing on charts) or otherwise breaking news kind of stuff.

Without that information, pilots will be unaware of dangers that may be involved in their flight.

VFR traffic was exempt from the stop, presumably because VFR flying relies far more on pilots' own ability/requirement to detect obstacles/trouble, and GA (which operate under looser regulations and may choose to accept more risk than airlines are allowed to accept on behalf of their passengers) were among the first to be additionally exempted prior to the ground stop itself being lifted.

But airlines are heavily regulated and aren't allowed to depart without reviewing the NOTAMS relevant to their flight path and if the NOTAM system is offline they're unable to meet that regulatory requirement.