Why do governments restrict imports ?

When governments practice protectionism (restricting or
banning imports), they are typically trying to improve their own domestic
economies.

Many governments feel that imports compete
unfairly with domestic products.  In developed countries like the US, governments often
feel that imports have an unfair advantage because of the low costs of labor in
developing countries.  Governments in countries that aren't so rich feel that their
domestic industries can't compete because industries in more developed countries have
had a head start in the race.  In both cases, this leads to a desire to protect domestic
industries from competition.  Governments feel that their own industries will then be
able to produce more goods and make more money.  This would, they feel, help their
domestic economies.

There are other reasons for
protectionism, but this is the most important reason for it.