World trade tends to happen via oceans. If you have a cost with a decent port, you can trade with a lot of countries at low cost. If you don't, you'll have to make arrangements to get your goods through neighboring countries to get to the port and that adds costs.

Wikipedia says Afghanistan has rail links to neighboring Uzbekistan and Terkmenistan, which are also landlocked; if planned rail lines to Iran and Pakistan are constructed, that would certainly help with access to ports.