The basis of modding Minecraft in any significant way is mappings, which are a translation between the gibberish names that Mojang obfuscates the game into before release into names that a modder can actually use and figure out how to do anything with. The problem is that every single release of the game means the names are usually scrambled to something different, so a new set of mappings has to be created, which is time-consuming work for a game as large as Minecraft. The end result is that a large amount of Minecraft simply isn't labeled, and sometimes the labels change between versions.
As of 1.7, the game became significantly larger, and 1.8 larger still. So each major game revision not only means it takes longer to convert the existing mappings, but the new code must be analyzed to determine its function to the best of their ability and labeled appropriately to make the new code usable as well. They don't even bother making mappings for every single release anymore due to the size of the game, it's just not very practical.
There are other factors outside of just the mappings as well, such as creating patches to repair the decompiled source which MCP and Forge rely on. Certain other things that were once vital to the process are no longer necessary to discover thanks to Mojang leaving certain information in the game as of 1.8.3 I think it was. But mappings will always be the crux which holds modding back.
As for 1.8, the community never moved to that for a variety of reasons, including what I would consider a boycott due to decisions made by the Forge team. Some larger mods are gradually stepping over the line, but I doubt 1.8 will ever be widely used for modpacks when 1.9 is already on the horizon.