The whole Excalibur mission was one part of a massive effort to find a cure. It was also an incredible long-shot. It was simultaneously the least-likely route to a cure, and the fastest if it succeeded. The hope is that it will stumble onto some abandonded First One world where information on a cure would be found, left over from some race that had dealt with the original Shadow plague. As a full-sized warship based in part on Vorlon technology it was the ship most likely to survive an attempt to trespass on such a planet, and it was staffed with the people best able to exploit any such discovery. No other ship would be able to boast such a concentration of talent as Excalibur, and it would be a waste to spread those people out over several ships. Gideon, Galen, Dureena, Max and Chambers between them were the best shot of finding, stealing, translating and adapting any data that was out there.
Regards,
Joe