There are many idol groups who instantly get famous after their debut. Whether it was because of their debut song, company or the members themselves, the public and the fans love them. However, not most of them maintain the popularity. There are many reasons why.
Here are some of those male idol groups:
BAP - We all know in 2012 that BAP was as famous as EXO; their fans even had a fan war for MAMA's best rookie boy group. During 2012, their concept was fresh and new and not your typical flower-boy type. Their debut song "Warrior", follow up "No Mercy" and "Crash" has cemented their popularity in less than a year. Many international fans love them as they are cool and very charismatic. However, after their long and destructive legal battle with their very own entertainment company TS, BAP is not the same anymore.
They did try and comeback with their old stuff, however, their fans had move on and are not that into them anymore. Also, there are many boy groups who have debuted during their long hiatus.
MYNAME - Myname is created by Hwanhee, the Fly to the Sky member in 2011. They were famous during their debut as they were created by Hwanhee and were called Hwanhee boys. They debuted with "Message" then followed it up with "Hello and Goodbye".
After debuting in Japan, they did not come back in the Korean music scene for more than 2 years. I guess that is why their Korean fans got tired of waiting. Also, like with BAP, there are many other boy groups who debuted in between so they kind of got forgotten.
NUEST - last but not the least, NU'EST. Nu'est also debuted in 2012. They are from Pledis Entertainment, home of Afterschool. They were also fairly popular after releasing their debut song "Face". Their members were well liked. However, after releasing "Action", they got into a behavior scandal plus they also debuted in Japan and China so they were not visible in the Korean scene.
I think K-Pop groups needs to cement their popularity in Korean first before venturing into Japan and China. They do get more money in those countries but it is still different when they are known in their own country.
With all the new boy groups, I doubt these three boy groups will be as famous as they were when they debuted.
