mosses are plants with very limited 'differentiated tissues'. this means that they only have a few types of cells. Like angiosperms consist out of roots, leaves, stem. etc. which all hold different cell types that perform different functions, the moss plant itself doesn't have any of that. The features and evolutionary improvement of them present advantages over their fellow angiosperms because they grow in the same part of an ecosystem. Mosses don't have roots they can't grow tall and win a battle for light with plants that do have roots.
