There has been thousands of reasons for the lack of manufacturers in the wrc but one of the most important reason is lack of succes. Subaru, Skoda Mitsubishi and Suzuki have gone primarily because they failed to win races.

Some manufacturers came in like wishfull amateurs without proper plan and proper resources. In the realm of Citroen and Ford you need really professional work with the car and even that's not enough. In rallying you need also top-notch driver. I think this was also problem with Mitsubishi. In the hands of Loeb, Grönholm or Solberg Mitsubishi would have been contender for race wins. In rallying real top-notch driver is huge factor (Imagine if Citroen had been without Loeb, Subaru without Solberg and Ford without Grönholm).