DIN was a smart choice for MIDI in 1983, but it’s a PITA today – it’s about the only thing that still uses DIN’s. Plus, the speed of MIDI is really slow compared to modern standards, and that affects important stuff like how long it takes a whole message to get somewhere. Plus PC’s don’t have DIN sockets.
A MIDI message is really just a piece of information. It can be carried by anything that moves information from A to B. Therefore modern MIDI devices send MIDI messages to and fro over USB, Ethernet, and other specialised systems.
It’s possible to take a MIDI message in from a DIN connector, and send it out again over USB. Similarly, you can take a MIDI message in from USB, and send it out via DIN. This is not technically hard, but it does introduce a small delay when moving the messages around. This is called ‘latency’, and too much of it in your system means you don’t hear the sound ‘as you play it’ – more than 50 milliseconds ( 0.05s ) in total is a problem for most musicians.