Well, partially it's a lack of enthusiasm, and partially because it's simply more difficult to get technical data on the hardware to emulate it. At least with consoles you can take the hardware apart and see what's in it and maybe how it works, but with arcade stuff you basicly have to rely on tech documents and pray you can somehow guess the right way to emulate it. Not to mention getting Roms to test the emulator with is very difficult (who has access to an arcade cabinate that's willing to take it apart and upload the rom data?).
But in any case, I really hope this project puts out a fully functioning Model 3 emulator in the end. It'll probably take them a while, but it'll be worth the wait if I can try some old classics like L.A. Machineguns and Lost World without having to track down an arcade machine (they were replaced by other games at my "local" arcade).
Oh, and it's interesting to note that even Gamecube emulators are currently more advanced then 90% of arcade emulators out there. Certainly goes to show how much easier/popular console emulation is.