Aside from the main problem of finding and keeping artists that can produce the quality of art a project needs, There are several other reasons that a causes a project to lose momentum, not all of them are true for every project, but I've seen them at work enough times to remember.
1- Too many members
A project moves at the pace of it's slowest member, specially if that person is one of the top ranks. People are lazy, they only work when they have to, and if there's one lagging behind, others don't see any reason to rush either. A small project has less people to slow others down.
2- Too much money
When the project is small, the team tries it's best to move forward and gain some attention while working side gigs to pay the bills, unfortunately when the money starts pouring in, the time that they're not spending on side gigs goes to spending some of the money and enjoying life, none of it goes to the project and often enjoying life takes a bite from the projects time too.
3- Too much have been done
When a new member starts on a project, they have lots of momentum, they are excited about what they are doing and want to see everything they ever wanted in the game become a reality, but not everyone on the team have the exact same vision, as they get a suggestion here, a guideline there and a criticism somewhere else, they lose their momentum to the friction of other members. The more material in the game and the more members on the team, the more there is to rub against.
4- Too many supporters
Often the supporters are passive and don't push the teams to move forward, specially on adult project where they don't want to get in touch with each other. With no pressure from the supporters, the developers don't see any need to leave their comfortable spot for the next milestone. With their big pool of supporters, losing few doesn't really make any difference.
5- They already got what they wanted
For some, the target wasn't finishing the game in the first place, they have planned from the start to amass a following and line their pockets as much as they can before the whole thing fizzles out. They have already got their target amount and are sailing the remaining wind to their personal vacation island.
6- Nobody else can do it why should we?
They know that nobody else has their files and consequently nobody else can take the project and finish it. They keep the rights to the art and the source files hostage to extract more money than they can ever get by selling the game. That's right, it's a hostage situation and they see themselves entitled to compensation for holding the unfinished work.
Well it took a real sinister turn midway, kill kill kill, burn it all to the ground, thigh high socks rule...! peace