Now that I think of it, there were plenty of R&B groups in the late 90's, early 00's.
That was my young adulthood, being that I had graduated high school in 1997 and turned 21 in 2000.
There were groups like Dru Hill, Blackstreet, Boyz II Men (although they were slowly becoming old school), Next (Nextacy was my shit back in 2000), Ideal, Ruff Endz, 112, Jagged Edge, Profile...
Then, the industry shifted and R&B groups became obsolete. People my age at the time came of age via R&B groups during high school such as R. Kelly and Public Announcement, Jade, TLC, Boyz II Men, Kut Klose, Mokenstef, Fugees, Total, H-Town, Hi-Five, SWV, and even solo acts such as Toni Braxton, D'Angelo, Faith Evans, Brandy, Monica, Aaliyah and Mary J. Blige...
But then, around 1998, that's when hip hop became about "bling bling" and everything about it seemed fresher and flashier... while singing about love and "Not Gon' Cry" seemed rather sedate. That's not to say, for a while there R&B and hip hop didn't coexist, but it was fleeting. When you're trying to appeal to a younger market, fresh and flashy is the way to go.
By late 2001 into 2003, that's when we started getting the influx of Eminem, Mystikal, Ja Rule, Ludacris, Nelly, Fabolous, 50 Cent, Chingy, Lil' Bow Wow turned Bow Wow, B2K... Ashanti?!
R&B sloooowly faded away.
Then Sean Paul came out, and then had to be featured on... Every. Single. Song. So, then, there couldn't be any groups due to the fact every song was So & So featuring Sean Paul. What's Her Name featuring Sean Paul. Whatchamacallit featuring Sean Paul. If you notice, all the music had to sound the same in order for Sean Paul to be featured on it. You can't really do a Jagged Edge "Let's Get Married" with their harmonies and have Sean Paul yammering through it and have it make sense for him to be on that track. So, you get a bunch of one hit wonders or even no hit wonders to sing a generic song that has a reggae/caribbean beat to it and have Sean Paul mutter the hooks.
Then came Usher and Beyonce in 2003... Usher, who was already in his mid 20s at that point, and Beyonce, while fresh as a solo artist in 2003, wasn't exactly 16 and unheard of.
So, the industry had to manufactuer the heirs to the thrown.... Let's use the Aaliyah formula and see if we can apply it to Ciara. Street but sweet, skilled dancer -- not exactly Aretha Franklin or Patti Labelle with the vocals. Throw in Bow Wow and make them the Hip Hop/R&B couple to watch... let them do a lackluster duet and when it doesn't really seem to do anything, they break up.
Next in line...
Christopher Maurice Brown and Robyn Rihanna Fenty... and we see where that wound up.
I think, after this, groups might start coming back in fashion.