Phaeophyceae or brown algae are a group of multicellular, photosynthetic algae that are primarily found in marine habitats. Here are some reasons why:

Marine environment
- Brown algae typically require a marine environment to survive and thrive. They are adapted to the unique conditions of this environment, including the salinity of seawater and the presence of other marine organisms.

Shallow depths
- Many brown algae are found in shallow depths, where there is enough light for photosynthesis to occur. Some species can grow in deeper waters, but they may need to be able to tolerate lower light levels.

Rocky substrates
- Brown algae often grow on rocky substrates, where they can attach themselves using holdfasts. The rocky substrate provides stability and support for the algae, and also helps to protect them from strong currents and waves.

Seaweed forests
- In some areas, brown algae can form dense "seaweed forests" that provide habitat for a variety of marine organisms. These forests can be important for ecosystem health and biodiversity.

Overall, the marine environment provides a unique set of conditions that are well-suited to the growth and survival of brown algae. While some species may be able to survive in other habitats, their primary range is in the ocean.