For the longest time it was just a fan ship based on the flimsiest of excuses. The main leads are on a team of 4? Well, Ruby and Weiss have actually interacted and their bickering happened at the height of the tsundere craze so they naturally got shipped. That left Blake and Yang, so they got shipped because they were the only ones left. Tautology and Pair The Spares!
Except for the fact that for like 2 seasons they had like 5 words between them. Season 3 or so bumped it up to a total of like 30 words! They had a conversation for once! Then the team gets split up for like 2 seasons spanning a few months. In that time Blake was coming to terms with her growing feelings for a different character, Sun, who’s she’s known since the first season and has spent his entire screentime flirting with her, and who pretty much became a main character during the Blake portions of seasons…4 and 5(?). So Blake already has a canon not-romance, and the reason it not blooming into am actual romance is due to Blake grappling with her fear over her murderous ex (Blake gets a lot of beaus across RWBY’s run. Her relationships being a microcosm are actually relevant to the greater narrative and themes at large believe it or not, but that’s an analysis for another time).
All of which gets tossed aside when Blake and Yang finally meet up. Nobody like S4 and S5 so Rooster Teeth made S6 (I think. I’m probably mixing seasons here) the fanservice season to “fix” the last few plots and rope back in fans. Which most relevant to this post is Blake and Yang suddenly being all the gay for each other when Team RWBY finally meets back up.
What about Sun? Exactly. He’s a nobody now, and I don’t think has been mentioned in years. Blake’s psycho ex and her reluctance to form bonds romantic or platonic out of fear of them becoming his target? He’s suddenly dropped into the plot out of nowhere, so Yang and Blake finally kill him together so they can all the relationship with each other now. Even though they’ve still only had like 4 interactions between them by this point.