This has been sitting unaswered for a while. The real reason is that there is no good answer.
Some folks might say that it's because gameotphyte-dominant species do not have a lot of vascular tissue that will support tall growht habits... So they could not easily grow tall... but they do have some vascular tissue. So in theory at least, one of them could have evolved the ability to be more tree-like. I don't know of any fossil evidence that this happened.
The bottom line:
Basically Natural Selection is a stochastic process. It has a lot of randomness, that cannot be reconstructed 450 million years later.