Aside from it being patented in part, the security goal of SRP doesn't
quite fit the way we use the Internet these days: it uses a procedure similar to Diffie-Hellman to
establish a secure channel based on the username presented. Meanwhile, we have a standard for
anonymous secure channels (TLS) over which we can exchange credentials without further crypto*, and using HTML forms means not being beholden to browser UI, such as HTTP Authorization's ugly modal dialogs
with no logout feature.
* Although it would be nice to be able do
<input type="password" hashmode="pbkdf2;some-salt" ...> to enable the server to store something other than a cleartext password, without all the dangers of trying to do crypto in javascript.
Bonus chatter: Someone once asked why I would use Digest auth even over TLS. "In case TLS is broken" didn't appease him, but since then, we've seen high-profile failures like DigiNotar and Comodo, and attacks like BEAST.
