Mass produced electronic circuit boards need to be manufactured in a highly mechanised manner to ensure the lowest cost of manufacture. The traditional leaded electronic components do not lend themselves to this approach.
Wired components were always difficult to place automatically because the wires needed to be pre-formed to fit the relevant hole spacing, and even then they were prone to problems with placement. It was reasoned that the wires that had traditionally been used for connections were not actually needed for printed circuit board construction. Rather than having leads placed through holes, the components could be soldered onto pads on the board instead. This also saved creating the lead holes in the boards which added cost to the production of the bare PCBs.
The idea for SMT was adopted very quickly because it enabled greater levels of mechanisation to be used, and it considerably saved on manufacturing costs. Although many connectors and some other components still require assisted placement, printed circuit boards are normally developed to reduce this to an absolute minimum, even to the extent of altering the design to use components that can be placed automatically.