I'm not going to act like NCT were physical monsters before 2020, but people were seriously understimating them. On 2018 with Empathy they sold almost 400k (this is GOT7 level by then) which were very good numbers for a group 2 years into their career. By 2019 both 127 and Dream were selling +300K easily but what truly made a change for NCT as a whole was Kick It; SM did a good promotion strategy for Neo Zone, delivering track videos for every single song on the album... and when Kick It happened it was crazy, NCT 127 was suddently charting the whole album on Melon (when it only was about streams) and Kick It got to be even 3rd on Melon with BTS as competition on some hours. Not to mention how they sold 750k the first month. Not to mention how insanely viral the song and dance got within the korean industry and even some locals. You'll find it dificult to find a more covered bg song from last year, specially on SK.
Apart from the album sales market booming, I'd say it's a lot of factors that made NCT stand from the rest the last 2 years when it comes to grow:
- Their daily videos channel, that somehow SM decided to abandon and separate their contents on their units.
- Very divisive music, I actually think is a good thing to attract fans, NCT music is the type to be loved or hated, which makes them a very hated group but also if you like NCT... you LIKE NCT, because they have a BRAND and you are very likely to support them through buying their albums or merchandise.
- Their impact on 4th gen, you just have to see the amount of 4th gen artists that are fans and look up to NCT.
Not to mention the impact the group has musically had on the industry, people who enjoy this type of sound know that NCT pushed it since the start and received heavy backlash from it, yet now it's the trendy thing. It also helps that a lot of their song sound ahead of time and have such a crazy amount of details and changes.