A cynical view of someone who has not participated in the arc community, and barley lurked longer then the time taken to sign up:
1) How long was it between you first hearing about Arc, and you seeing the first prototype? For me, it was a long time. Expect similar changes to happen along similar timeframes.
2) Arc is a Lisp: a language notorious for community fragmentation and trivially incompatible implementations. Without a strong, active leader, expect more of the same.
3) Arc isn't so much a prototype language, as a mini-fad / hype-based language. All things which survive find a purpose, Arc has rejected practicality as a goal, and anyone doing language research has probably already written their own Lisp dialect, so the market is probably limited to Hacker News readers. Activity on arclanguage.org probably spikes whenever it comes up on HN, and depending on the news, has a long or short trail off. I probably won't return until another interesting Arc headline reaches to front page.