Bryophytes never formedforests (mats maybe, but not forests)
because
a) they possess flagellated sperm.
b) of bacterial parasites
c) they lack lignified vascular tissue.
d) they have no adaptations to prevent desiccation.
e) their rhizoids were too weak.

C) they lack lignified vascular tissue. ( Since they do not have vascular tissues, they are not able to develop vertically like the trees we know.)

Explanation:

A) Plants do not have "sperm". They have sporophytes or gametophytes.

B) No specific parasitic bacteria of bryophytes are known.

D) In fact, they have. Although they require water for their reproduction, some species of bryophytes are found in deserts.

E) Rhizoids are not weak, otherwise they would not have the ability to keep bryophytes attached (mosses).