Like many have said, few people seem to be willing to touch the Python code, which means most mods are dependent on a single scripter, which means it's an all or nothing affair. Warhammer mod is a good example; I'm the only one scripting for it, and I have an RSI related injury plus a full time job to juggle, on top of the usual social life et al. On top of this, I've had several occasions where I've just managed to get something almost working, and Armagan goes and chucks out a new version which either breaks, or else offers a much better way of achieving the same thing (like bloody villages). It's hard enough to get the scripting part working as is, without a race against the clock thrown in too.
Maybe the hardest part with scripting is that it can't be shared out very easily. The python files all impact on each other to some extent, so it's not like I can say to you - "you do the scripts_py, and I'll re-write the simple_triggers stuff...."
One person has to do the scripting virtually solo....
Not necessarily, the way the scripts are designed one could easily farm out the troop editing, item editing and the like, and simply copy & paste into a file when you're done. In my own experience, these relatively simple parts are as time consuming as trying to get a more complicated script working. Conversations for example; any total conversion would need the entire dialogue (or most of it) changed, and a good 90% of that has little to do with actual scripting, it's just altering the actual lines people are saying to make sense in the context of the mod. Even having someone simply work through and change the phrases, while leaving the actual script alone, would save a huge amount of time for something like Warhammer. Yes, there'd be problems due to the inter-referencing, but I'd rather waste an hour or so doing a quick check that the files are correctly cross referencing than spend six hours creating 30 or so troop profiles from scratch (especially if it's something like a troop referencing a wrong item; that kind of thing should be spotted during testing and is relatively trivial to fix anyway). The problem is finding people who are willing to take a pop at it. I'm sure we could get enough modellers to give every single person their own model to work on, yet as soon as you mention anything ending in .py nobody is interested.
I think the community has a part to play in this too. I've seen several occasions where someone has proposed a mod idea, and has simply been shouted down. There's two problems with this:
Complaining about a lack of scripters is all very well, but this situation isn't ever going to change unless the community does something about it. All budding scripters have at the moment is a bunch of guides by Winter which are around four versions out of date (and incomplete), and the various discussions which have cropped up in the mod forums (from which it's pot luck whether you'll find what your looking for, or anything remotely useful to your own set of circumstances). Needless to say, it's not exactly the easiest way to pick stuff up. Nonetheless, there are plenty of people out there who are capable of scripting to a degree, even if it's simply adding your own troops or items.
More guides would of course help. Or allow would-be scripters to participate in a portion of the script work for your mod, even if it's simply adding troops or the like. A good project for any scripter with time on their hands would be some tutorial mods - a mod with some minor, documented changes to the script to allow people to experiment with making their own alterations.
The second is the tendency to flame people who simply propose mod ideas. Like I've said before, if you have nothing useful to say, don't bother posting. In the first case, we'd be better served by encouraging them, even if you're not interested in the whole project ("You do x,y and z and I'll show you how to do a & b"). Also, people shouldn't underestimate the use of a project lead type position. They're essential in professional software development for a reason. I think some of the mods which fail, especially the larger projects, might have fared better with someone co-ordinating the mod. A good project lead will provide encouragement to the team, will set the short term goals the team should be aiming for and will also know when to push for something. They can also co-ordinate the feedback from releases, keep track of where the mod is currently act and keep development on track. They're especially important when you have a larger team.