I think there are many sectors where just one or two companies really dominate the industry.
PayPal seems like one, especially for international payments. iPhones and Androids. Windows. Cargill for food.
Granted, I am having trouble coming up with companies where "just one" company dominates the market, but overall this seems more common than one would like to think.
You are correct in that "it is difficult" would not be sufficient to explain why there are so few competitors. But I think I feel similarly about these other industries as well.