There are many reasons to why any new members will lose interest, most times it's not them losing interest, they seem to have find the answer they are looking for and they will be less active, until something crypto related comes up again.
There are some that real life happened to, they don't have the time to be active anymore, when life hits hard at times the forum will be the last thing that you will remember.
Right now crypto market isn't looking that good and many have already capitulated their portfolio, this is humans for you, some will be willing to be a part of Bitcoin race while others will get weak very quickly.
The most popular part is making money through the forum, when newbies hear this they become encourage but after they start using the forum they lose interest easily because things doesn't always seem like they pictured.