This is a symptom of our current system. Anybody who has any ambition in politics will modify some of their positions to fit one of the major two parties. If running for another party wasn't a death sentence, you would see more competent candidates in those parties.
Which is worth doing because, regardless of what the ideal situation is, if it's associated with a spectacularly bad outcome in one of the earlier elections that uses it, that's going to be a big impediment to wider adoption.
So the minor parties (and independents) are eliminated first, and their preferences are distributed. Which usually means than one of the major party candidates scrapes over 50%, reflecting the minor party voters' basic preference for one of the major parties over the other.
In this (common) situation, I think IRV actually works really well.
The major parties still end up in government, but they see the first-preference percentages for the minor parties, and understand what policy positions are important to the electorate.
With IRV, though, I could see a situation where they are much more likely to win simply because, nowadays, Republicans' and Democrats' general level of mutual disdain is such that I wouldn't be surprised if it ends up being common for people who identify with one major party to rank the candidate from the other major party below every other candidate more-or-less out of spite.