French leftist parties collude with French centrists

In France, the leftist and "centrist" parties are having candidates drop out for the second round of voting to stop the conservative parties from winning an outright majority. Why would the centrist parties do this with the left?  Either they are leftists themselves, or they are being spiteful about their loss, or they realize that they will disappear if there are only left and right candidates holding the vast majority of seats. They don't want to be irrelevant. France may yet succumb to the self inflicted status quo they don't want because of this, joining the U.K. in the sadly, seemingly unstoppable drift leftward that has plagued most of western society, despite the growing desire to stop it.