Because they theorized that there must be a particle that acts a certain way? Then they observe this particle and it matches their expectations.
It’s a pretty big deal when a theory predicts something that has never previously been observed. Then, once you know what the theory predicts, you go out, look for it, and find it.
Einstein predicting that gravity would bend light, which turned out to be true when tested was a big deal. So is this. The standard model, as gawky and ad hoc as it seems, makes good predictions of things we have never seen before and has done so pretty consistently.