The short answer to your question is that mosses (there are many species) were around before vascular systems evolved. Some of their relatives evolved into vascular plants like our modern ferns. As you know, once they had tubes to carry water and nutrients up, and carbohydrates down, they could get a lot taller than mosses.

You seem to be wondering, "If those tubes are so great, why didn't mosses get them?" The answer is that natural selection does not work that way. You don't get something just because it would be helpful.Mutation (change in DNA) is a random process. If you're lucky enough to get a helpful mutation, you're more likely to leave more offspring than your neighbor, but getting a good mutation is just luck.

Does the lack of a vascular system mean that mosses aren't successful?Not at all. Mosses have been around for over 300 million years. That's a pretty good track record. They can't live everywhere and do everything that a vascular plant can do, but they do some things better than vascular plants, like living in cool, moist, acidic environments that don't get a lot of sunlight.

Can you think of any other plants or animals that are pretty much the same now as they were when dinosaurs were roaming around the earth?Would you say they are more or less successful than a species that is relatively new? How would you decide to measure "success?"

Mosses are the descendants of plants that never evolved vascular tissue, much in the same way that fishes are vertebrates that never evolved arms and legs.