A few things at play with YB's popularity:
-The South is very supportive as a region, and since most black folks have roots in the South, YB has popularity on all coasts. Especially in cities with prominent street culture (Philly, Detroit, Chicago, Los Angeles)
-Youtube. I dont think people understand that a vast portion of Gen Z exclusively browses youtube, discord and twitch. This means kids were able to consume YBs music, for free, yet because the bulk of his listens are on youtube, he largely has no Billboard success or monster sales/streams
-Hes a handsome kid (

) and personifies the archetype of "toxic, yet tragic, sympathetic and passionate hood nikka who cant be tamed" so women love him
-He's sort of a drill rapper? And Drill has become this weird sub-culture where people become so fascinated with the warring factions that they build parasocial relationships with the artists. His feud with Durk and Von really elevated his profile (and durk and von are also hood popular in the same way YB is)
-Lastly theres been a linear path of baton rouge nikkas blowing up from regional acts to national acts (boosie -> gates -> YB). What makes YB different is he massively benefitted from being so young when he blew up, the rise of youtube and social media, and so far, not letting incarceration fukk up his momentum like it did Boosie.
The Execs at UMG mustve saw all the data from YT/DSPs/SoundCloud and realized hes a unrealized national act. Couple that with the current musical tour bubble we're in, figured that there will never be a better opportunity for him to headline a national tour.