We know that forest consists of tall trees. In the tall trees the water and minerals are transported to the every part of tree through the lignified vasucular tissue. This tissue not only helps in transport within the tree but also provides some sort of mechanical strength due to the presence of lignin it its walls. On the other hand bryophytes are non-vascular plants which means they lack vascular tissue and thus a mechanism for long distnace transport of water and minerals is absent in them. So they can't be taller as a tree and thus can't form forest.
