Very short article here, I highly recommend the book it's based on.

In short, Trotsky was just one guy who did good work in the revolution and then fell off afterwards for ideological reasons, and became the face of a tendency with an inherent social basis. Even with no Trotsky, there would still be Trotskyists, they'd just be named something else (probably after some other person specifically, the tendency kind of needs a 'heroic perfect martyred theorist' as its focal point).