Not sure but usually it can be because you already have reached the limit of what you can earn from multiplayer but you can still earn ninja pearls from finishing the mission quests like Heart elements only etc. or earn 1 pearl for getting all the drops on a certain stage including the lucky drop. Hope this helped :)
I feel that for the free to play players its nice to have this but its also unfair to limit it. I don't spend money on the game and the people who do are going to be that much better its very annoying. if we were to still gain pearls I think I would play the game more often because right now its just boring. There is no new campaign missions or anything on global so I can't get more pearls. What you should do is you should renew the 110 pearls every new update for us free to play players.