There were a few factors making letsencrypt successful: alternatives were not free, and mainstream browsers were all making attempts to surface unsecured http c

There were a few factors making letsencrypt successful: alternatives were not free, and mainstream browsers were all making attempts to surface unsecured http clearly on address bars. A free open source alternative for captcha will have a harder time getting traction, mainly since the popular option (Google) is already free; and neither business not customers (who are not privacy sensitive) care about the end result unlike the browser and http case.