Because reefs are poorly-studied and not as 'well protected' by laws as terrestrial areas are.
Nearly 60% of Earth's coral reefs are threatened by human activities such as development along waterways, overfishing and pollution. Additionally, Coral reefs are strained by rising ocean temperatures and increasing ocean acidity levels.
Reefs are important recreational areas for many and are loved even by large portions of the public who have never had the opportunity to see their splendor firsthand.
Coral reefs are sometimes referred to as the “rainforests of the sea,” because they teem with life and abound in diversity. But although only a small number of Americans have ever had rainforest experiences, many more have had the opportunity to dive and snorkel in nearshore reef areas.
And in contrast to the obscured diversity of the forests, the gaudily colored fish and invertebrates of the reef are there for anyone to see. Once they have seen these treasures, the public becomes transformed from casual observers to strong advocates for their protection.
For more info, see this Microsoft Word Document: Coral Reefs Affirmative - Saint Louis Urban Debate League.
TIP: Check out NOAA's 25 Things You Can Do to Save Coral Reefs.