The greatest concern of terrestrial animals is breaking or maybe drying out. The creatures inside also need more support for a longer time. There are probably many small reasons for it.
Aquatic species, frogs, fish, amphibians, are have other problems. Many of them are coated in goo so that they will stick to plants and rocks and not float away. The structure must also keep the nutrients for the offspring from leeching out of the egg into the surrounding water.