TBH, this happens with just about everything. You have three groups of people who noticed this project when it was first announced: people who were interested, people who weren't interested, and the people who are kind of in the middle. The biggest group is going to be the middle group and they're the hardest to keep into the game. Just think about how many people played and downloaded Reborn Episode 13 vs Episode 15. I know one big factor was Shofu playing Reborn which created a spike to people playing the game. People are going to stop playing it for many reasons such as burning out/losing interest, forgetting about it, or something else more interesting comes down the line. It's often why I tell people to wait until you've got quite a bit accomplished before announcing a fan game because people really want to play it then which the sooner they can the better since people interested will drop quickly the longer it takes.
Instead of looking at the total numbers of people leaving or kept going until the end, percentages would be more accurate and comparable, but both of these take work. The second season will more or less really tell how successful it really is since people like to try new things and this won't be new any more. Basically, what I'm saying is take the number of drops in this season as a grain of salt since it happens in almost everything. I wanted to get that out of the way before giving you the answer you really wanted:
The Reborn league also kind of had it better since it was on the main channel and there wasn't much competing with it like the fangames and the forums (which have grown...I think). A certain few drama moments didn't really help, but they didn't really affect the numbers that much. The availability is imo very poor due to it being a time where people are busy with work if they're in school or straight up have other things they need to do (I rarely had a free saturday since like 3rd grade). It was 4-7 Fri, Sat, and Sun right? That's 12 of 168 hours in the week or 7% of the week. If everyone shared the same timezone, this would be a minor issue, but we don't. The other thing is if someone got stuck on a leader, they had two options: start all over from scratch or just quit. The latter is probably the more preferable choice nowadays since there's plenty else to do. Of course, this is just me being theoretical so I could be wrong. The time thing was a big reason why I didn't participate in the league.