Answer:
Because they are triploblastic ( Triblastica ).
Step-by-step explanation:
Their body is composed of endoderm, mesoderm and ectoderm. While Cnidaria are Diblastica, so they are composed only from two layers: endoderm and ectoderm.
In addition larvae of Echinoderms are bilaterally symmetrical and they become nearly-radial only as adults.