There were several factors that played into the popularity of these. Harley suucceeded, but any movie about their origins will make it sound like this was their original idea.

Harley had a great deal of competition specifically because the bar or entry was so low at the time. Anyone could start a "company" in their barn/garage, and many did so.

At the time of this model-1, there were kits that you could attach to a bicycle. Harley felt there was a market for a stronger frame and a more powerful engine.

Ford introduced payment plans in the 1920's, so before then, vehicles were a full price cash purchase. Cars were the plaything of the rich, until the Model T, which sold for under $500 just a few years after it was introduced.

It would be difficult yo convey how popular bicycles were from the 1880's until the Model T, and motor kits for bicycles were hugely popular.

Bicycle manufacturers began making optional frames that were stronger, and were pre-made to accept a motor kit after the owner had some time to save up for the purchase. In this way, the bike and the motor could be purchased separately.

WW1 wiped out hundreds of small motor-bike companies, with Indian and Harley surviving. As with many other aspects of our past, the winners write the history books.

Harley was simple, rugged, and cheap enough to survive, and just powerful enough and advanced enough to be desirable.

The Flying Merkel was much more advanced, but also too expensive for the mass market.

Harley recognized that racing was good advertising, and that low price was vital to survive business cycles, just like the Model T.