I'm starting with GLSL shader programming and have been looking into RenderMonkey. Sadly, AMD no longer supports it. Why? Is there a successor to it?
The bulk of this answer is going to be a response to the "Why?" part of your question, alas.
Well, there's FX Composer, from NVIDIA, which is a similar product -- it doesn't support GLSL but the languages it does support are quite similar. But it was last updated in 2009 and I know of no plans to update it further. Most high-end 3D modelling packages have material construction tools in them as well, which may or may not support GLSL.
I think the reason you see these products reaching end-of-life (and nothing coming out to replace them) is that the direction that shader development has taken doesn't lend itself well to generalized IDEs like this.
Even back when we just had vertex and pixel shaders, there tended to be a strong coupling between the game/engine-side data formats (and how that data is processed) and the shader input layouts and operations programmed within the shaders -- at least for the more interesting, complex effects.
Consider water-related effects, for example, which often involve deforming the geometry slightly in response to sums of sine waves (in addition to running a computation that sums similar waves into a texture to be bound to the pipeline as a bump map), as well as needing framebuffer copy textures to bind in order to simulation reflection, and so on. Most of that data comes from the CPU, and isn't built in to the shader itself.
As the nature of that coupling increased (due to increases in parallelism on the CPU side, allowing us to balance the interesting computations more evenly between CPU and GPU), it got harder and harder to design a general-purpose shader IDE because that IDE also has to have a way to script and replicate the data pipeline that a game is going to send to shader -- essentially it needed to be a plugin to an engine. As we added extra programmable stages to the pipeline -- geometry shaders, hull shaders, et cetera -- this only compounded the issue.
D3D tried to alleviate the problem with SAS, but I don't think that ever really caught on and it certainly hasn't scaled with advances in GPU technology.
As a result, specialized in-house or in-engine tools for building shaders evolved, and tools like FX Composer and RenderMonkey fell into disuse and were ultimately abandoned.