Why do countries like France waste tax money on two-round elections?

[BolivarianHero]

Why not just have instant-runoff voting and decide it in one go?

[hobbit709]

1. Maybe they know something about instant runoff.

[Selatius]

2. It might require a constitutional amendment to do something like that over there.

Their constitution probably outlines how elections are run, and changing it would probably require a hefty amount of political capital. Unlike the federal system where you have three layers of government (local, state, and national government), in France you only have the local government and then the national government. Technically, there is a middle layer called "departments," and they are sort of like states here in the US, but they are under the national government's authority and not really independent.

[malaise]

3. Because they don't waste it on advertising

or buying candidates.

[FarCenter]

4. The candidate with the biggest plurality in a 3 way race may not be the preference of a majority

Suppose candidates A, B and C got 40%, 30% and 30% of the vote in the first election.

It is possible that in the second round, B can get 60% and A can get 40% if candidate C's voters all prefer B to A.

Voting schemes have a lot of problems and don't always (or maybe don't often) wind up electing the person most preferred by the electorate.

[Ozymanithrax]

5. Instant runoff doesn't allow peole the time to think about the remaining...

candidates in the election. Why do we have primaries. Why not let everyone who can manage run for President. Imagine if there had been 4 or 5 Republicans, 4 or 5 Democrats, and various independents running. The one with the most votes becomes President, and the one who comes in second becomes Vicepresident. That was how it was supposed to be before they changed the Constitution.