The way I picture it, there's kind of a tendency decision tree as you move further left. If you go down the Marxism tree, you eventually get to the point where you have to wrestle with Stalin's legacy. If you move on from that point (rather than stopping or going back a step), you can go forward in the decision tree with or without Stalin. If you choose Stalin, you go down the Marxism-Leninism tree, and without Stalin the main tree is Trotskyism. (This is just my mental picture of it, not the way I think it actually is, just in case it needs to be said lol)

Back when I was a baby leftist, it was mostly just a matter of "I like the Soviet Union, and I like Lenin, but not Stalin, since I've been told all my life that he's basically Hitler" and at that point the natural choice is "well after Lenin it was either going to be Trotsky or Stalin, and I've already ruled Stalin out, so let's go with Trotsky". Thankfully that was just a stage I went through along the way, but I figure some people get stuck there and become Trots