Reliable, good quality, fuel efficient, high resale value, lots of models to choose from, etc.
Many people do not know that Honda is the more reliable brand with an even higher resale value, better quality, more fuel efficient cars, and better performing cars.
The first Hybrid in America was the 2001-2006 Honda Insight, which got 66mpg highway. But it was a 2 seater hatchback so it was not popular and Toyota got the credit that Honda deserved because the Prius was a bigger car and there were lots more commercials for it.
And Toyota has been slipping in quality. Ford caught up, Chevy is catching up, Mazda caught up, Nissan caught up, Subaru surpassed it in quality.
Now most cars have caught up to Toyota in fuel effiency and quality, but back then, Toyota and Honda were the best so they have the reputation of being the best.
Most people still think American cars die around 100,000 miles and have bad gas mileage. But all cars can now make it to 200,000 miles and all car companies make some fuel efficient cars, but they do not have that reputation that Toyota and Honda have.
If the American's/German's/Korean's/British started off making good cars like the Japanese did, then they would be equally popular to Honda and Toyota.