Rhyming words doesn't seem like a skill that would fade, the opposite actually, I would think it would get better with age
So what happens? Just a lack of passion?
Since the beginning, hip hop has been a successive series of "eras" with their own particular sound and style (Disco; b-boy; pro-black; gangster; boom bap; shiny suit; bling; crunk; snap; hyphy; trap;). Its proven very difficult to transition to the next era when you're associated with the old one. Hip hop fans are notoriously fickle, and move on quick.
Then you have personal issues which cut a career short:
Substance abuse is obviously an easy one.
Run-ins with the law. Big one. Shmurda couldnt even capitalize on his record because of his legal issues. Remember Shine? How big would he have been if not for that bid?
Young black males obviously have a lot more drama going on in their lives than the average person, let alone the average artist. Therefore they are easily distracted and pulled away from the studio. Cant focus.
Pressure to either build on success or duplicate it. If you had a hit or two, the label wants you to reduplicate that. If you showed promise, the label wants to you to break thru and produce hits. This sort of pressure can damage the creative process, and often leads to writers block and shytty results.
Label politics. Labels forget about you, and dont prioritize your next project because they're focused on newer and more promising act (in their minds) they want to put their money and energy behind. You always hear about albums being shelved or pushed back and this is usually why. This goes back to the initial statement, as a label might think fans have moved on from your era and sound.
Ego. A lot of niccas get big heads and start shytting on people left and right and make enemies. When that artist need favors or help, the people he dogged and fronted on aint trying to hear him. Foxy Brown is a great example...
Just a few reasons....