It’s a phenomenon that has been studied by psephologists and political scientists. Ultimately, it’s because IRV is a single-winner system. And single-winner systems have a strong preference for 2 party systems because at the end of the day, if one person wins, it’s likely to be from one of the largest groups.

Our Senate doesn’t use a single-winner system. It uses STV, which is a proportional system. Unfortunately because we only elect 6 Senators per state, there’s not a lot of room to create proportionality, so there are still only a relatively small number of groups represented. Contrast with the MMP system used in New Zealand and Germany, or direct proportional systems like the Netherlands and Norway, and you get much better truly proportional results.