It's hard to say for certain, but I can make a few observations…
The two stages, "Definition" and "Commitment" serve very different purposes. In "Definition", any interest in the site is still very much an abstraction; a bunch of people waxing philosophical about "what would this hypothetical site be about?" That part is easy. The second part is the more difficult. That's where people have to get real and "commit" to using this site they designed. So, two possibilities: either the followers weren't the hard-core, committed audience you need, or they simply lost interest over the 3-1/2 months it took to drift over the line into Commitment.
Either way, don't despair. A "Dermatology" site is squarely within the demographic of what will make a great Stack Exchange site. But perhaps our current generation of Stack Exchange users aren't going to be the ones to build it. Now is the time to renew efforts to reach out and develop support for a proposal that's all-but-ready to go. Serious users have such an easier job of it now. They just have to want to show up.
Another approach is to tap the resources and brainstorm with the Area 51 community directly. We are in the final stages of grouping proposals into "Categories" and adding meta discussions specifically for each of these more-focused groups. The benefit to you is that you can ask communities that have roughly the same interests as yourself. Perhaps together you can figure out what is slowing progress and how to best rally the support you need.