Most likely, the answer is ROI. Implementing SRP costs time and money. What do you get for what you pay? You could as well ask why SCRAM (RFC 7677) is not used, while we're at it. Or why client X.509 certificates are not more widely used.
I believe you need to perform a bit of threat analysis to understand this. (From now on, I am assuming secure network communications provided by TLS.)
Question: In which case do better password protocol provide additional security?
Answer: Better password protocols prevent illegitimate use of the password if the server is not trustworthy.
Question: But, if the server is compromised, why should you worry about keeping your password secret?
Answer: Because the same password is used to access other valuable resources.
But wait... this problem has been solved by password managers since the late 1990's (Schneier - PasswordSafe) or perhaps even earlier!
Now, let's look at another threat: let's assume the client system is compromised.
Question: Does SRP mitigate this threat?
Answer: It does not.
Question: Are solutions which provide mitigation against this threat widely used at this time?
Answer: Yes. (Google Authenticator, Blizzard Authenticator, SMS verification codes - provided the smartphone is not the device used to access the valuable resources - hardware tokens...)
Thus, I'm afraid the answer is there is little incentive to implement better password protocols.
EDIT: I was about to forget this! Dumb password protocols do not leak the user name (still assuming ordinary TLS) while TLS-SRP does. Thus, to sum up the answer: no existing threat mitigated by switching to TLS-SRP and another threat is no longer mitigated.