Question

Why echinoderms are closely related to chordates and hemichordates?

Answer 1

Actually they are both deuterostomes

Deuterostomes are distinguished by their embryonic development.
In deuterostomes, the first opening becomes the anus, while in protostomes it becomes the mouth.

Some claim that all deuterostomes come from the same deuterostome ancestor.
The fact that they are, indeed, both deuterostomes is why the phyla Echinodermata and Chordata are grouped together along with Hemichordata