I have beta tested a number of SM and PM toolkits/applications and have never been impressed by one of them. I even tried my hand at writing a stage management application - but after a while, I lost interest in the idea (partly due to the fact that I could not define exactly what the application should do).
Almost any programmer will tell you that the core of good software design is the knowledge of what the software will do. Now - define what goes in a prompt book. It is entirely subjective and is defined differently by each stage manager, and changes based on a number of factors - including but not limited to the show, the director, the crewing situation, the venue and the mood of the SM at the time.
So to make a great stage management appliaction, it needs to be extreamly configurable. But if you make it extreamly configurable, then usability (generally) decreases, and the setup will be more involved - especially when you take into account that it will most likely need to be customised on a show by show basis.
So to approach from the other angle - design a tool that does 'generic' SM stuff. It may work great for one SM on one particular show, but the second the SM wants to do something outside of the box, things start to fragment and data loss becomes a major problem, and in the end you would be better off reverting to the old fashioned way.
The final angle I can see is to create applications that fill a niche in the SM world - however those applications often already exist to fill rather generic needs - timers, calendars, attendance software, report writing tools.
There is also another driving factor. Profitability. You will never make your fortune off of Stage Management software. The market isn't really there, neither stage managers nor theatres have the money to blow on the software either. So there is no incentive to develop commercially.
Most theatres I know manage their inventory in excel, they do their accounting through an off the shelf accounting package (or excel) etc.