This likely isn't too mind-blowing for many, but I enjoyed this answer to the question - 'Why do artists often produce an initial, fantastic album, but then lose popularity'
https://www.reddit.com/r/hiphopheads/comments/1neo58/why_do_rappers_fall_off/
This is eye-opening for the writing I do myself - I've spent about 10 years accumulating inspiration and honing pieces, but if you asked me to produce something of equal quality within two years - next to impossible.
https://www.reddit.com/r/hiphopheads/comments/1neo58/why_do_rappers_fall_off/
One thing that hasn't been mentioned is that artists spend pretty much their entire lives up to their first album, writing that album. The Weeknd in his recent AMA said he had been writing and perfecting House of Balloons for years to the point where he had multiple completed versions of each track and just picked the versions he liked best. Kanye had been stashing instrumentals for years away for The College Dropout. I guarantee that Kendrick Lamar had the at least some of the ideas floating around in his head for GKMC and Section.80 since 2004 at the latest. Nas' entire life experience to that point was summarized on Illmatic. etc etc.
Now I'm not saying Kanye, Weeknd, Kendrick, Nas fell off after their first work, but it proves the notion correct: artists spend 20+ years writing their first album(not just the literal writing, all the experience, practice, honing, training etc that goes into it). Then they are expected to drop a new project every year or two and it is hard to conjure that sort of artistic inspiration on a dime sometimes. Obviously there are tons of exceptions(many of whom I've named already).
Another issue I think that happens with a lot of rappers these days is almost over thinking it. They are starting to blow up off mixtapes, earlier albums, and then release a new album that really strays away from what got them hyped in the first place. Whether it is being too overproduced(Wiz Khalifa), too many pop attempts(J Cole, Yelawolf) or just a natural change in style(Earl), rappers can overthink it and lose part of what made them great in the first place. I'm not saying all change is bad, and certainly Cole out of those rappers has found a pretty happy balance on Born Sinner with radio stuff that still feels more natural, but it isn't always executed well.
I almost feel the rappers that never change are some of the most immune to falling off. Curren$y pretty makes the same type of album/mixtape every time he drops something with little variation. And he is consistently good. Ghostface Killah, same deal. You can listen to tracks from Fishscale, 12 Reasons to Die and Supreme Clientele back to back to back and while there is definitely differences, none of it sounds too radically out of place. Big KRIT's album may not have been as good has some of his tapes, but he pretty much does similar things every project. I feel like guys like that, as long as you like their schtick, aren't gonna fall off as much as say someone like Cudi who constantly does crazy out-there shit that can be more hit or miss.
This is eye-opening for the writing I do myself - I've spent about 10 years accumulating inspiration and honing pieces, but if you asked me to produce something of equal quality within two years - next to impossible.