When I learned about SRP we were told it wasn't seeing much deployment due to possibly infringing on EKE patents. Network Computing had this to say in 2002:
Standards groups have made several attempts to induce Lucent to talk about its EKE patent -- to no avail. Even with Lucent's silence on the topic, few vendors have been willing to use SRP. To further cloud the situation, there's been a new patent issued claiming that it too covers SRP.
The SRP website, however claims
Since SRP is specifically designed to work around existing patents in the area, it gives everybody access to strong, unencumbered password authentication technology that can be put to a wide variety of uses.
If I had to guess, I'd say patents were part of the original problem and that effect has continued even today. Whether patents are the sole reason for limited deployment or even the major reason is beyond me.