Why did Sparta discourage trade?

A Spartans first and only vacation was to be a warrior, anything else would be an insult to the other Spartans. The job of trade went to the Perioikoi, they traveled all across Greece and the Aegean and made Sparta fairly rich.

Spartans could travel if they wished, usually they just didn't. The city of Syracuse in Sicily was founded by Spartans. Travel was however discouraged as it tended to be the case that when a Spartan left Sparta they came back with all sorts of 'bad' habits.

They were not allowed to trade and travel because they could not risk a war or battle to start on foreign ground.(Even though they were a military community)