Question

why some organisms are considered as diploblastic

Answer 1

When there are two layers of germ in the blastula of an animal they are considered as diploblastic.
The two germ layers are called endoderm and ectoderm. Jellyfishes and Corals are two such examples. But other higher forms of organisms have three germ layers in which mesoderm is also included thus helping them form organs.

Answer 2

see the attachment for answer