Hi - Cardinal flower can be susceptible to root rot problems and your description of the roots being very short/barely there points in the direction of this being the problem. Cardinal flowers like to be in consistently moist soil with a pH range of between 5.8-7.8. If the moisture or pH is off, it is possible that the soil conditions were favorable for a root rot pathogen to take hold of a stressed plant. Also, cardinal flower is typically a short-lived species; it has a biennial habit -- meaning it will bloom in the second season and then seeds and dies. The plants have to be re-seeded to maintain the population. The affected plant material should be discarded in the trash (not compost). You may want to do a soil test of this area to check the pH and nutrient levels. https://extension.umd.edu/hgic/topics/soil-testing

The Missouri Botanical Garden has noted that some of they hybrid Lobelias have not performed well in their trials, but the reasons for that are unclear. Research-based information on disease problems in this species (and its cultivars) is limited.

Christa