The changelog includes the result of such changes but not the detailed reasons. Details such as screenshot comparisons are in the forums. Sometimes they are in the thread for the individual mod but often they are in threads about the mod that is replacing them or in threads comparing a set of mods (this is usually the case for mods in the Conflicting Graphics section of the guide). You can search the forums for these comparisons; for example using advanced search look for "Skyrim HD" including the quotes.
SRO was originally chosen rather most of the textures in Skyrim HD several years ago, but SRO was removed from Nexus and some textures from Skyrim HD were added back to the guide. Now that an improved version of SRO is available on a reputable site it was added back to the STEP guide since a comparison review of these SRO textures vs. some of the other mods with conflicting textures preferred the SRO ones, and as a result some mods were removed from the guide.