The Meaning of Life, the Universe, and Everything.
A lot of people hail 1.7.10 and 1.12.2 as the best version for modding, but nobody seems to remember that 1.6.4 was just after the birth of Forge, and as such many mods were made for that version. 1.6.4 is also a little nostalgic for me, as there is relatively old worldgen and the game is still pretty simple at this point, allowing lots of room for a wide variety of mods to be installed.
The 1.6.4 minecraft version is one of the best versions of minecraft for forge. It has unique mods, for example, the more explosives mod, wich is very nostalgic for me. So, I agree with you, sure, there are tons of good versions out there, but 1.6.4 is one of them.
Not only do I still play in 1.6.4, I have never updated to any newer version, and I develop my own personal mods for it, most notably "TheMasterCaver's World", a comprehensive mod which changes just about everything, including adding some newer features (usually not the same though, like how Mending directly replaces renaming an item); the next version will include more than 300 additions and changes (up until now TMCW had mostly been a world generation mod with relatively few new blocks and items, many of the new additions are new blocks, items, and mobs), even some that will be in 1.14; aside from this, most of my gameplay is in more or less vanilla with only a few minor mods (e.g. changes to regional difficulty, "hard only" features on all difficulties, a couple blocks that enable crafting rails and cobwebs into storage blocks) and bugfixes (e.g. zombie pathfinding lag). I've even exploited the fact that 1.6.4 can't download a custom skin anymore by replacing the default skin and player head textures with my own (I did this long before as insurance against server outages, now it is like my signature; in TMCWv5 the player head will always be my own even if you use your own custom skin using the provided resource pack).
I'm so old school that I don't even use Forge for mods; I just use MCP and directly edit the code using Notepad (no IDE like Eclipse, even for a mod which currently has half as much new source code, not counting modified classes, as all of vanilla), which is far more efficient than using Forge (I've never been able to understand how any mod needs even 1 GB of RAM; TMCW only uses 100-200 MB, even less in the upcoming update despite hundreds of new features; FoamFix, an optimization mod, claims that Forge with just itself runs worse than vanilla due to how Forge uses memory). This is also because of the difficulty of changing vanilla code (e.g. I asked how to replace the cave generator with no replies, during a time when the forums were near their peak activity).
The reasons why I still play in 1.6.4 are due to the changes to world generation in 1.7, which nerfed the underground by making cave systems much smaller and less dense and mineshafts and dungeons much rarer; my old computer also had a strange performance issue on any later version (even 1.7, which did not have the major lag and other issues of 1.8+, had a weird stuttering as shown here, with no known cause or fix. Even on my current computer 1.6.4 far outperforms newer versions, especially 1.13+; I've made some major optimizations in TMCW, especially the upcoming version, which make it up to 10 times faster than 1.13 in some cases (they even added a loading bar to 1.14; why bother though if the game launches in a second or two?). 1.6.4 is also MUCH easier to mod and optimize since the code is a lot simpler based on what I've seen). A lot of it is just because of the way I play though; TMCW is really more of a fun side project to show how I might update the game than a way to add features that I want (I've taken a break from updating it for the past month and have been exclusively playing on a world which is vanilla 1.6.4 except it has double the cave generation, based on an early modded world I had).