The main reason for that was because they focused their attention to maritime trade and thus haven't developed strong army as they were trying to preserve good relations with their neighbors and the outsiders.
Explanation:
Phoenicia was regarded as one the trade centers. They had good connections with Egypt, Mesopotamia, Crete and were the first ones who went outside the Mediterranean sea. Thus they managed to established a large number of colonies, among which the most famous one was Carthage.
