steve64

That's a serious over simplification.
I'm not taking the time to research the weather at all airports involved, but this soooo sounds like a fuel issue that's being caused by a secondary issue with weather and/or the fact this is TGU we're talking about.

As MSPeconomist theorized, it could be as simple as a fuel shortage (or contamination/etc) at TGU.

Or...
The fact that the scheduled flight distance is well within the range of the aircraft is usually irrelevant in "day of" flight planning. After all, the aircraft is the one scheduled to fly the route. But to legally dispatch a flight, the airline doesn't simply have to ensure there's enough fuel on-board to fly to destination. They have to (and this is still an over simplification) have enough fuel to fly an approach to a runway at the destination, execute a "missed approach", then fly to an alternate destination. The designated "alternate" can't be any airport ... it has to be an airport where the weather forecasts at arrival are above certain minimums. In other words, there has to be enough fuel on that plane that the airline knows they can find somewhere to land, irrelevant of the weather. Throw in bad weather across the southeast USA, now DL and AA are hosed with extra fuel required to dispatch flights to ATL/MIA.

So now the airline has enough fuel on the plane to meet the "alternate" requirements. At takeoff, is the plane now too heavy with all the fuel? For most planes and airports, especially the flight length of TGU to ATL/MIA, the answer is an easy "yes". But operations at TGU are never easy. The airport sits in a "bowl" surrounded by mountains. The airline must ensure the plane is light enough that if one engine were to fail close to liftoff (more technical .. beyond the point where the plane can still stop on the runway) and still clear the mountains with a margin of 2000 feet.

Range is is least concern. They gotta be light enough not to hit anything on takeoff, yet have enough (heavy) fuel on-board to know they can land somewhere/anywhere with decent weather.

UA still showing normal operation to IAH is irrelevant to what DL or AA are doing.
If IAH weather forecast is great, then UA's alternate is probably HOU.
If the southeast is socked in, the alternate for ATL and MIA might be ... IAH.
Such alternates do happen. In my years at AA Ops, some I can always remember are:
DFW to Miami ... alternate Savannah
DFW to Toronto ... alternate Nashville
DFW to Cleveland ... alternate DFW
Now we're talking some serious, legally required fuel loads on flights that should be well within the plane's "range".