I can speak to this a bit. My mom is a lawyer and we've talked before about the need for better templating systems. The problem is that the market for solo practitioners and small firms is just not that large or lucrative, and AIUI the market for large law firms is already pretty mature, or at least there's a high barrier to entry.

But there's definitely a need among solo practitioners and small firms, and that need goes beyond templating to include better ways of organizing documents, automating workflows, ensuring security, and so forth. There are companies like Practice Panther which are trying to do this, but the problem I've seen with them is they encourage vendor lock-in and make it hard to do anything not officially supported by the platform.