I think the calm disposition of the Drafts, came from their history of being work horses. Men of old, wouldn't have kept or bred, horses which could likely spook or run away pulling a plough, or going through the bedlam of cities, with a cartload of beer for the taverns.
Equally, carriage horses were favoured and bred for their flashing way of going and appearance. Small ponies were originally bred for their sure-footedness and obivous size.
All early horses had a job and were bred for the job intended. Just like dogs were bred for a job. Only more recently have all the different breeds of equine been kept for actual riding and pets. Therefore, we find different types, work ethic and temperaments.
The main reason my daughter first decided upon the Gypsy Horse, was because of their temperament. One of her mares, was the youngest horse to ever take part in The Rose Parade and that with only 30 days training. The Rose Parade is a huge test of temperament, considering the millions of people lining the route, with giant floats, kids throwing things and very loud bands. The mare didn't make a wrong move.
Certainly, careful breeding plays a big part and should today, just as in days gone by. Sadly these days, not everyone considers temperament first and we see some rank animals around and producing more of the same.