T-Pain was cool in 2006 because autotune was "new" in hip-hop at the time. What he did was special. But once everybody started getting the plug in, nobody needed him anymore. Artists like Roscoe Dash, Lil Wayne, Lil Durk & Kanye showed that literally anybody could use it.
Same reason you never hear songs feature actual R&B singers to do hooks. The artists can do it themselves and save money.
Nearly every T-Pain and Akon feature was actually a reference tracks shopped around the industry with their hook already on it. That's why it's multiple artists rapping on the songs they were on the hook (Shawty, I Wanna F**k You, etc). They became cookie-cutter.
He's some early versions of songs that were released by other artists.
Plies fell victim to the exact same cookie cutter formula of women songs with a singer on the hook.
He used that formula so much that for his 3rd album he signed his own singer (Chris J) in order to the hooks.
Problem was people had gotten tired of it. And the next single "Want It Need It" featured Ashanti, who was also in a similar creative rut.
Keep in mind that Plies released his first 3 albums within 14 months. The debut was released August 2007. The 2nd album was released in June 2008 and he released the 3rd album six months later in December 2008. They struck while the iron was hot, but it made his formula too obvious.
He waited and dropped his 4th album in June of 2010 and it was pretty much over besides "Becky" and some his features like Wasted with Gucci Mane & Lost My Mind with Jeezy.
The promo song for the album was yet another girl song, "Medicine" with Keri Hilson and it went absolutely nowhere because it was the same song he always released. They didn't even put it on the album.
So basically they played themselves out and people got tired of them. Plies was able to get back poppin as an IG personality. T-Pain never recovered as an artist. Both of their runs get neglected.