Because it just does:
The three IRV countries:
Ireland (mandated in their 1937 constitution),
and Malta (and more recently Fiji
for a brief period of IRV democracy before its coup)
all are 2-party dominated (in IRV seats)
– despite having many other features in their governments
which would seem much more multiparty-genic than the USA with IRV added will ever have.
So you can be sure the USA with IRV would be 2-party dominated too.
"The composition of IRV seatholders in Australia is very biased against
third parties: examining all 564 statehouse & federal IRV seats we found only a
single one occupied by a third party."
Because here is an example election
and here is another in
which, if some voters honestly order the candidates N>G>B where N=Nader is the "third
party" candidate, then N and G both lose, but if those voters
dishonestly vote G>N>B or G>B>N, then G wins (a better result in their view).
In these example elections, just like under the present plurality system, voting Nader
is therefore strategically foolish even if he is your honest favorite.
A possible reason why the Australian
House remains 2-party dominated after 80+ years of IRV is that
these kinds of elections happen
often enough so that strategic voters feel logically justified in thus "betraying"
Nader. Hence: the third parties suffer
a tremendous disadvantage, hence they die off.
And hence voters observing this year after year
realize the third parties have no chance, which justifies
their strategic vote-exaggeration/betrayal decision all the more.
(Who cares if you betray N if he had no chance anyhow?
It is worth it if it increases the chance
G will win – or so they reason.)
Result: Vicious cycle – entrenched 2-party domination – third parties die.
We do not know that this is the actual reason, but we will say that
it would be entirely logical if it were; the only way it is not the actual reason is
because of the insufficient-logic of Australian voters. If we instead assume
more-mentally-challenged voters who don't know or can't understand this, then they
will likely just give the two-most-likely-to-win candidates top & bottom rankings
– without worrying about whether
that really is logical or not – to "max out their vote's impact."
That is intuitive and requires no thought at all
(but nevertheless is supported by at least some deeper analysis)
and with IRV will prevent a third-party
candidate from ever winning.