Your scenario is one that actually existed, so you may wish to concentrate on (#1) what may have caused it in the past, and (#2) what could constrain it in an arbitrarily specified world, such as yours, where you determine the rules.
There are many causes of transition to land among plants that appear fortuitous, in the sense that the specific adaptations (functions that allow conquest of land) may have appeared in different forms, but for luck and historical contingency. As it turned out, the principal limit was probably time. One could refer to this as a non-equilibrium theory. Evolution is clearly a stochastic, non-equilibrium process at long time scales. Specifically, (#1) some time had to elapse between the "invention" of photosynthesis (plants) and functions that allow plants not to be submerged in water (land plants). During this period, all living plant lineages had a shot at coming up with dry adaptations, e.g. cuticle, adjusted gas exchange, aquaporin expression, etc.; and later vasculature and seeds (prevalent in "vegetation like grass or trees" from your scenario).
Otherwise (#2), you can come up with any number of constraints that could plausibly prevent the evolution of drying out and forming tall structures. This is what Josh King and ArborianSerpent have in mind. Some of those could be arbitrary limits on what plants can do in your physical/abiotic setting, and some could be biotic limits (e.g., animals in your world are pre-adapted to eating plants; microbes are pre-adapted to parasitizing them).