Question

Why do you think eggs of a frog do not have a hard covering?

Answer 1

because a frog is big and the space is needed is a lot

Answer 2

The eggs of frog are covered with a hard shell because they need moisture to grow . If there were a hard covering outside the egg, the eggs would have dried up. Instead, they have a jelly-glycoprotein like covering that keeps the eggs moisten.  

Secondly, frog lays their eggs in moisture so the jelly like formation helps them to hatch out easily.