Hi, Ray. In answer to your question, I don't think it's that it's particularly difficult, but rather that it's hard to know what to do. While the general public has really started migrating to X now, that's pretty recent. Even a year ago I would have advised people to think about it before jumping.
The big problem is that in the graphics industry, the switch to X has been much slower, primarily because Quark is still the industry standard, and they just got around to announcing Quark for X this month. But there are a LOT of problems, supposedly, with the X version, and even then what makes Quark work is the third party X-tensions that let you customize your work flow and none of those are available for X yet. I've only heard of one maker who's even announced they plan to port their Xtensions to X.
So if you are a graphics pro, you may be stuck in mac os 9 (even in the sheet music business, I am--no finale for X), so for a plug-in developer it's still a big headache deciding whether to do it for X or for the old os. I don't blame people for just avoiding the issue.
Also, truthfully, I think the popularity of X caught a lot of developers by surprise. They figured it would either never get off the ground or take many more years to become the preferred OS for the average mac user. I'm pretty sure that's what happened with Finale. Their annual report for last year acknowledges that nobody much bought Finale 2003 and it seems to finally be sinking in that people just won't buy OS 9 software anymore.