[EM] Why does IRV but not delayed top-two runoff lead to 2-party domination?

[Jan Kok]

The statistical evidence at http://rangevoting.org/TTRvIRVstats.html
seems pretty good that IRV leads to two party domination in IRV
elections, while (delayed) top two runoff tends to lead to a strong
multiparty system.

Why do those two methods, which seem strategically quite similar, lead
to such different results? The above mentioned page has links to some
speculations/explanations, which I find less than convincing. The main
proposed reasons are:

1. Different strategy calculations by voters under the two systems.
Voters who like a "third party" candidate seem more willing to vote
for their favorite in the first round of TTR, than corresponding
voters under IRV are willing to rank their favorite 1st. Why, why?!?
Most IRV supporters in the US have no clue that voting their favorite
1st can ever hurt them. From my limited discussions with Australians,
it seems most of them have no idea either. So why aren't Australians
voting for third party candidates as their first choices, enough that
they might occasionally win? While at the same time, voters in TTR
countries feel free to vote for whoever they want, often enough that
TTR countries tend to have flourishing multiparty systems?

2. Under IRV, if a "third party" candidate makes it to the last round,
it draws little attention from the media. But under TTR, when there is
a runoff, both candidates get equal attention for several weeks. This
lets the finalists compete on an equal basis, so the third party has a
realistic chance to win, and even if hse does not win, the party and
candidate will be remembered in the next election.

So, is either or both of those the real explanation(s)? Is there some
evidence to back up these explanations? (I suppose statements by
Australians, Irish, etc. saying that third party candidates rarely get
any attention from the press, and statements by Argentinians, etc.
that third party candidates _do_ get a lot of attention from the
press, would be helpful.)

It would be nice to put together some convincing and preferably
non-technical explanation about why IRV leads to two-party domination
and TTR does not. The purpose is to persuade IRV supporters to leave
US TTR elections alone, or convert to something better than IRV.

Thanks,
- Jan