Because of the way that glaciers and waves shape the land.

In the polar regions, glaciers advance and retreat over thousands of years, carving out fjords and other features in the coastline. The glaciers erode the land, creating valleys, and deposit the rock and sediment they have picked up in the process, building up landforms like moraines. This process creates a crinkly or jagged coastline.

In tropical regions, waves are the main agent shaping the coast. The waves erode the land primarily through a process called longshore drift, in which waves hit the coast at an angle and push sediment along the shore. This process creates a smooth coastline because the waves tend to erode the land evenly, not carving out fjords or creating other distinct landforms.

Climate and sea level changes also play a role. In warmer climates, sediment is transported more quickly, leading to a less-pronounced coastline and sometimes sediment deposits forming barrier islands or lagoons