In FS1, space travel, or combat for that matter was nowhere near what is was in FS2. Space travel/combat evolved in those 32 years, much to the thanks of the Shivan threat at the speed like computers today. In FS1, cruisers were the mainstray of the GTA and PVN fleet, think of the costs in manufacturing destroyers, thrice the size of modern day carriers and motherships. After the Shivan invasion and the formation of the GTVA most likely more money was put into military research, much of efficency and technology was most likely researched from shivan technology etc.And the tech room does say that Cruisers were used for escort duty.