It takes a lot to make an plugin or editor.
1. First, to provide a good quality plugin, you would need to procure several major DAWs for Windows/Mac to validate the code since not all DAW handle plugins the same, and I speak based on personal experience. Some DAW only support VST2 and not VST3 and VST2 is not officially supported by Steinberg anymore, VST3 is the way to go, but has major MIDI limitations that VST2 does not have The there are 32 and 64 bit version, so a lot of time goes into testing
2, Buy Windows/OSX software licenses.
3. LEarn the development tools and how to code a plugin/editor
4. Cost of the synth itself
5. The development platform, some VST development platforms ask US $1000 for their tookit / license if you sell the product. if it is developed with their free toolkit, you can charge but the source code should be released (something not seen much on Win/Mac platforms). License terms vary per toolkit. Sure, some people sell plugins developed using these toolkits, but perhaps not meeting the license term, which is not politically correct, but because it does not eat into a lot of the /income from toolkit providers and legally problematic to go after the pundits, it's been tolerated.
6. The price of the plugin should be low to get some sales for those limited number of people who will use an editor.
No wonder, not many folks are interested in this bussines, there is little gain to develop editors, unless you do it for fun.
DSI could outsource it, but that may not be cheap as well.
BTW: DSI provides excellent support for questions and quality documentation even there are some mistakes and missing details. Just check recent MIDI documentation from other manufacturers and you will know why: Many manufactures (Roland/Nord/Moog)documentation are far much worse, don't document at all or even omit Sysex implementations nowadays, few even use NRPN. and this is getting worse with every new product as manufacturers deviate from the open available MIDI standards for patch loading/saving etc... Just look at the Roland System 8. One of the main reasons I bought DSI stuff is of it's consistent MIDI implementation. Most other manufacturers should take DSI as an example how things are done right, MIDI wise.
I would be happy to make an editor if sponsered by an OB6 and P6 desktop :-) even knowing that people won't donate a penny if it's for free even they just bought a $2000 synth.. But, in the end I think I will end up looking into my pockets for some $ to get these instruments. without a physical unit it's hard to make sure to provide quality software. P.S: I am not affiliated with DSI directly or indirectly.