1) The path specified by VSTPluginPath in the registry
2) C:\Program Files (x86)\Steinberg\VSTPlugins
3) C:\Program Files (x86)\VSTPlugins
When it finds a folder, it will assume that all other VST plugins are in the same folder and stop searching.
My best guess is, that iZotope installed in folder 3) and then they worked. When you installed Waves they were installed in folder 2) and then the iZotope plugs were hidden.
In order to get them visible again, you need to reinstall the plugin or manually move the DLL files to the same folder.