The reason is the way Neytan works.
Other devs often works in this way:
for example the dev releases a version for $20 tier , then after a week for $10 tier, after a week for 5$, after a week the update is public.
Neytan releases version for $20 tier, then he continues to develop the plot, so the version for $10 tier will have new images and new animations; again Neytan will continue to develop the plot, so the version for $5 will have new images and new animations; the same for $3. So it is impossibile a public version.
Maybe when version 0.8 will be released, the final version 0.7 might become free.