I believe the answer to that question lies in the fact that mosses and liverworts have an entirely difference composition and makeup.
These types of plants lack the usual characteristics of other woody or flowering plants. They lack stems, deeply lobed or segmented leaves, a vascular system, ovules, and seeds. Instead, they reproduce by means of small spores. The spores send out filaments which grow into a matted, clump-like carpet similar to thin, green felt. The mat eventually produces specialized cells that form distinctive male and female parts. You can observe these as little antenna-like projections rising above the clump or mat. A full reproductive cycle will produce more spores, which in turn propagate the species.
Most mosses rely on the wind to disperse their spores, but some species produce special leaves and branches, which when broken off, can reproduce with being fertilized.
Mosses can live on rocks, trees, wood, and by the sides of streams. Wherever they occur, mosses require moisture to survive because of the small size and thinness of tissues, lack of cuticle (waxy covering to prevent water loss), and the need for liquid water to complete fertilization. Some mosses can survive dessication, returning to life within a few hours of rehydration.
This, then, is the foremost reason why it is restricted in size and habitat.