Why Diameter is almost irrelevant.
The IETF has spent a lot of time standardizing Diameter, which was supposed to be the next-generation successor to RADIUS. For a while, I was sure they were right. It turns out that Diameter doesn't matter as much as it could have.
For one, the Diameter base protocol is huge. The OpenDiameter people have been doing a good job of implementing it, but it's still much more complicated than RADIUS. That complexity has a cost: no one implements it.
Where a NAS vendor could spend a few days and have a pretty crufty (but working) RADIUS client implemented, the cost of implementing a Diameter client is much higher. As a result, almost no NAS vendors offer Diameter support. And since there's no support, there's no demand for it… which is a vicious circle.
The main benefits of Diameter are network features like guaranteed delivery, and automatic server discovery. Since the Radiator people cam up with RadSec, many of the benefits of Diameter can be obtained via RADIUS.
That effort hasn't (yet) translated into a RadSec implementation for FreeRADIUS, but it may happen soon. Once there are a few RADIUS implementations that usurp Diameter's functionality, there's even less reason to do a fork-lift upgrade to Diameter. In the end, I have a hard time seeing Diameter taking off anywhere, which makes it pretty much irrelevant for most people.