What holds them back?
Are there specific features that are missing?
Or is it just a usability thing?
You have to keep in mind that retouchers are generaly working for many photographers until they find a powerhouse to stay-in. They need to be really good at one software that they know it will be a standart whatever Windows or Mac will be used. They were generaly trained in schools where the software has been PS since the early days of digital imagery.
The good trainings are given on PS. With PS they can't be wrong, so they don't even bother learning another soft because on a job interview they will have to PS.
I see a similar situation in the motion industry. Here, if one really want to join a high-level training structure, it's falling into: Avid for cuttin, Smoke for power app, and Nuke for FX. You train on those, there are jobs almost guaranteed if you're not bad, and generally the reputated structures and teachers are there. There is not really a hig-end training on a Premiere Pro or Vegas. Go to the national teevee training and it smells Avid from all pores. FX cine is Nuke and Maya. There is almost nothing on Blender, and Blender is good (what the Gimp is to PS), but Etc...
It's all about what a specific industry considers as a standart, and the schools use the softwares the industry asks for.
People would be amazed to know how many printing houses are still working on FreeHand despite it's no longuer existing for awhile. Of course they know ilustrator or in-design, but as Free-Hand was a standart, and a well appreciate one, there is absolutly no prob you send free-hand files, even today. I do it all the time, and that's why I kept a XP machine because Free-Hand is not suitable on windows 7.