Because the math doesn't work. Most "next-gen" captcha fundamentally fail (by orders of magnitude) one of the many pillars that make captchas scale....
1. Is it trivial for a human to answer correctly? This affects growth.
2. Can humans do it quickly? This affects growth.
3. How is the random guess-rate? This better be abysmal.
4. How good is the “opposing” technology?
5. How is the guess rate of a sophisticated attacker, using said technology?
6. How much human input is required to create your captcha? You better be asymptotically better than human-solving the captcha.
7. What are the cultural and accessibility issues?
I remember suggestions of using computing power to slow down guess-rates. Probably related to bitcoins. However, it doesn't work since some users don't seek better computer performance.
1. Is it trivial for a human to answer correctly? This affects growth.
2. Can humans do it quickly? This affects growth.
3. How is the random guess-rate? This better be abysmal.
4. How good is the “opposing” technology?
5. How is the guess rate of a sophisticated attacker, using said technology?
6. How much human input is required to create your captcha? You better be asymptotically better than human-solving the captcha.
7. What are the cultural and accessibility issues?