why is BTS so popular compared to other groups?
They had a very strong run of EPs/albums with The Most Beautiful Moment in Life, Part 1, The Most Beautiful Moment in Life, Part 2, and Wings.
Run was probably the turning point: it was generally an amazing song, but it was also a part of a series of videos that looked to be connected with a single story. That improved overall engagement with that run of videos.
There was a general growth in k-pop in the United States, partially due to Psy's breakout in 2012, partially due to YouTube (and other services like Dramafever and V Live), and partially due to events like KCON. So, BTS had a series of songs and videos that did well on YouTube, headlined KCON LA and NY in middle of a great run, and in general, engaged with their international fans (especially on social media) in a way most groups did not at the time online. (For example, SM Entertainment has been real slow to have English subs on YT videos and very slow to allow artist engagement on social media and video platforms.) And that's before getting into other stuff, like strong hip hop influences starting to take off.
So it's a lot of factors. Nothing wrong with that. BTS is riding a strong wave. That doesn't mean other groups are doing very well: Bigbang is mostly in the military but even their solo singles hold the charts in Korea, EXO is strong in Asia, girl groups like Blackpink and Twice are very strong in terms of streaming, and we don't know "what's next".
K-pop is great, man. Let's enjoy the ride, especially since it means more acts coming Westward.
If you're interested in a general history of K-pop stuff, here's a solid video on it.