shorasLoL was developed together with creators of original DotA. At the time it was released it was pretty much the only modern MOBA.
Learn some history.
The only real developer to move in with Riot for League was Steve Feak, better known as Guinsoo. There was another developer, Pendragon, but he didn't actually work on the game itself - he made the original forums, and helped foster the new League community. Guinsoo was not the original creator, either. He forked it from a guy named Eul, and lots of other people forked it too. Guinsoo took all of their ideas and pulled them into one blanket, calling it DotA Allstars. He threw feature after feature in and, for the most part, the balance was abysmal.
Then, after two years (and a few other developers sprinkled in there), the reigns of development were passed on to Icefrog, who did all the actual balance work, making the game what it is today over the course of half a decade.
Guinsoo went on to Riot and Icefrog went on to Valve. Neither was the original creator. One of them, however, has the notoriety for taking other people's ideas.
At the time it was released, it was the only modern MOBA, correct, although Dota followed shortly thereafter (and was known to be in deep development before League was released). HoN also followed between League and Dota, having also been in development during that time period. It's not like it was the only one being worked on - it was just the first to release.