That is a really good question, one which is open to alot of debate. Like you said, it probably would have been very espensive to have armor plating put onto the Lusitania during construction when they didn't actually know a war was going to break out. Also perhaps they thought at the time that nothing could do enough damage to her hull to sink her (just a theory). As for why Mauretania was chosen for service and Lusitania wasn't... perhaps they just needed one ship for service at that time. Going to check into it...