How many primaries have hundreds of candidates in them?
The California recall election had 135 names on the ballot.
But that's rare though. In the California senate race this year, there were only 24 names on the ballot:
http://en.wikipedia.org/wiki/United_States_Senate_election_in_California,_2012
Granted, that's still a lot. But it seems cheaper and easier to just have one IRV election with that many names on the ballot than to have a separate runoff election.
Easier for poll workers perhaps, but not voters. To make IRV work, you need to keep the number of choices within reason, either by making the ballot requirements stiffer, or by making a partial IRV system that might still lead to runoffs. (I.e. voters get to number their top five choices. If no one gets a majority, the top five show up in a runoff.)