morgans originated in your area, as a versatile working horse. Probably why there are alot of them. Just like TN walkers are all over TN. Different regions have different horses.
Endurance has specific rules that favor arabian traits. There just isnt any way around that. Although Normally cooler in Vermont they recent vermont 100 if I recall correctly was hot. It is all about heat management, pulse rate and recovery. All things being equal an arabian will simply cool off quicker, and pulse down quicker than any other breed. A super conditioned walker, morgan, or other breed can do well but you are starting yourself off with a handicapp. Go to tevis and look at the horses that have finished in the last 10 years. Look at the ones pulled. I did the math once and 90 percent of the finnishers are at least 50% arabian. That remaining 10% consists of horses that may have arabian as well, breeds like mustang, or grade.
The crosses that show up most frequently as competitive seem to be the breeds I listed above. People are competitive, competition is a crucible. That which works rises to the top. That which doesnt is cast aside.