I'm guessing desktops were significantly cheaper, and could therefore start from the bottom where people were younger, didn't really care or had other issues (and didn't have secretaries to start with). The price of early laptops meant only higher-up execs were able to get them, and those were generally older men who had had secretaries most of their life (or who had just had secretaries attached to them, these secretaries becoming a status symbol of having "made it")
