Fortnite blew up after PUBG because PUBG, while novel and well-executed, was PC only and ran like ass. Fortnite ran better, was F2P, and most importantly was multiplatform. I don't think that it's success was a surprise at all, actually. Apex Legends took Respawn's control and gameplay mechanics, which were always top notch, and brought them into this genre where people could see them.
For Helldivers 2, the game went viral, but I think it's tough to figure out why. It doesn't do anything new, unlike PUBG when it came out. It has the same gameplay structure as the first game, which was popular but definitely not some massive hit. It doesn't have amazing graphics or mechanics, although both are fine (the engine it uses was discontinued years ago FFS). It has a tone that is a callback to popular movies, and it doesn't have wokeshit, but that's not going to make people play a game they don't want to play.
But it really, really hit with people. I am tempted to chalk it up as one of those random viral games like you see on PC every once in a while, but it has more legs. Whenever that happens, you can usually point to many similar games that just didn't land and it's tough to distinguish why.