Why don't weather forecast models normally take solar eclipses into account?
Total solar eclipses are rare.  Globally, they only happen every 18 months.  In any given spot, they are much, much rarer, with a recurrence period of many hundreds of years.

Solar eclipses are localised in time and space.  Although model resolution may be just about small enough (in time) to resolve the totality, it only has a limited impact.

One could implement solar eclipses in weather forecast models, but there's plenty of more important things to worry about improving.  Operational weather forecast models are complex beasts, so you really want to change things only if it may yield a significant improvement.  The potential improvement of implementing a consideration of solar eclipses is minimal, so it's simply not worth the effort or the risk.

Experimental models are a different category and as you have seen, eclipses have been implemented in those.  But the risks involved are much smaller; nobody really cares if experimental models go totally wrong, but you really don't want to be responsible for breaking the operational weather forecast model.



You might be interested in this article on The weather’s response 
to a solar eclipse that  my colleagues wrote.