Why have the avenues of exchange they provided been open only for limited time spans & then blocked for years or decades at a time?

The avenues of exchange they provided been open only for limited time spans and then blocked for years or decades at a time because of War.  

People at both ends of the trading route have to secure and keep that area peaceful or else trade will not occur.

answer; no longer than a reasonable period of time;

answer; pluralist democracy;