Why are echinoderms unable to survive on land?

because they have some specialities to which they can only survive in the water such as they begin their lives as free swimming bilaterally symmetrical larvae etc.for e.g:star fish,sand dollar,sea-cucumber etc.

Because they did not come to land. First one to come to land are amphibians but they are going to water for reproduction. Reptiles are the first form to fully adapt for terrestial environment.