Why does the French election have a runoff election instead of preferential voting

Question

In the recent French election, there will now be another vote between Macron and La Pen to determine a winner. A lot of other countries seem to have this system. Elections are not cheap and having another vote will make it twice as expensive and making voters vote twice will lead to lower voter participation.

Why does a country adopt a system like this when they can just use a preferential voting system like in Australia where voters can just specify who their votes will go to during the primary vote if their initial candidate does not win, meaning there is no need for another vote.

Answer 1

When you have a bunch of candidates, it can be hard to decide between them. For example, in the Republican presidential primary in 2016 (in the United States), there were so many candidates that some were never really considered. And even among the top candidates, people didn't spend as much time on their second choices as they did on their first choices. Like Jeb Bush? Who's your second choice? Rubio? Cruz? Kasich?

Two rounds means that in the second round, people can really look at both candidates. For the many people who don't vote for them as their first choice, this may be the first time they've really considered these two candidates.

It's also worth noting that multiple rounds and preferential voting are not mutually exclusive. They could use preferential voting in the first round and then a straight vote in the second round. That would give more weight to compromise candidates and less to extremes.