Because of "micro" transactions - These games made most digital sales in 2015 from AmyStephen's blog

According for your latest report from survey firm Superdata was League of Legends in 2015 the overall game with all the highest mmoah grossing. Total sales with the digital sales increased by 8% compared to to never long ago.
League of Legends undisputed number 1
Project-League-of-LegendsThe MOBA Riot Games is leading the ranking together with the top-selling free online games of Superdata which has a clear margin. Estimated 1.626 billion US dollars the ability rinsed those responsible inside mmoah.com till. Only three more games could exceed the magic level of 1 billion US dollars in sales also each year ago: two from the PC, that happen being especially popular in Asia (Crossfire and Dungeon Fighter Online) and one for that mobile devices (Clash of Clans).