Good question Allison....

The reason why is that they all run on the same computer, (or computers) and you can't really run them concurrently...so you have to arrange them so that forecasters in the field get the data they need as quickly as possible.

The base runs of the models are set up off of the rawinsondes (weather balloon) launches at 00z and 12z. That data is critical to the models, so they have to run at those times. Then the models are sorted by 2 criteria a) which one is needed most by forecasters and b) which ones run the fastest. The Eta is the primary US short range model, and is used by every forecaster in the country, so it goes first (after the RUC, which a fine-scale model that runs every hour and only out to 12 hrs...it doesn't take long). Then the old NGM (shaking my head), then the AVN/MRF.GFS suite. The GFDL and other hurricane models have to run after the GFS because they use it for the basis of their forecast.

I hope this answers your question...at least a bit....they CAN'T run at the same time, so they have to be prioritized.