The Star League sourcebook attempted to give an in-universe explanation - that Star League-era medical research was primarily focused on battling alien plagues contracted by humans on thousands of colony worlds, placing older maladies on the back burner as far as talent and resources were concerned. The Succession Wars didn't help the medical research community.
That being said, many of the disease treatments created during the Star League subsequently became LosTech. Notably, League doctors were able to cure Rhean Marik's cancer, but centuries later, the First Prince of the Federated Suns, who should have had access to the best care available, died of cancer, implying that the treatment for Rhean was either LosTech, or wasn't applicable to his type of cancer.