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.
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