As a rider gets older, it is common that they might gain weight and lose some flexibility and as a result need to raise their handlebars higher to compensate. A longer head tube allows for the stem to go higher without going past the limit line or look goofy sticking out so far if they are going to use a Nitto Technomic. In the classic era with level top tubes, it was expected that the rider have their handlebars lower than their seat. So a mixte frame has 2 advantages. It not only has a longer head tube than a diamond frame of the same size but a rider can ride a bigger one (with a longer head tube) since there aren't any straddle issues.