Most of the voters aren't TOs who know better. Custom moves are a huge logistical nightmare and it's just not worth the hassle. There's a difference between thinking they would be cool to use in tourneys and being able to efficiently organize a tourney that allows them.
I've also noticed from scrolling through this thread that 80% of the posters have 2014 join dates and under 100 posts, as if they've made accounts just to promote custom moves.
Looks like it's time for my OP forum stats to do some work
I'm gonna start off with the fact that I am for custom moves(and by extension Mii Fighters), but against equipment, just so everyone knows where I stand.
So... for the most part, arguments against custom moves can be boiled down into two notions:
1. It's way too hard to implement the usage of custom moves in tournament.
Most naysayers against custom moves will talk about how difficult it is to get them working and ready to go for tournaments, since it's hard to unlock them on WiiU. As someone who's trying to 100% the game, I will admit that it definitely is difficult to do this, but the transferal of custom sets from 3DS to WiiU is most certainly not. Moving through the menus and getting a set over from one system to the other takes no more than one minute to complete. And... just like making custom controls for names on systems, sending your custom sets over is a one and done deal that only needs to be done once. As soon as you've imported a set to a system, that system is good to go for the rest of the tournament. With custom sets implemented, it definitely wouldn't be any kind of hazard to time constraints and the like. It just doesn't take that long.
And some people might say, "Oh, so now you have to spend all of this money in order to enter to competitive scene?" C'mon, though. How hard is it really to just ask someone who's got the goods on 3DS to transfer it over when you are unable to since you don't own one? This is probably what people would do regardless, even if the concern were not to be addressed in any way.
One other concern about having custom moves legal is that your opponent will be kept in the dark about what customs you might be using. This is definitely an issue, and would have to be addressed in official tournament rules. To alleviate this, a simple addendum of "By request, you have the right to ask your opponent about what custom special moves they are using," to rulesets is an easy enough fix, I'd say.
2. Custom moves negatively impact the game.
The best way to start to approach this argument is to see if there are any inherently broken custom moves in the game. From a preliminary examination, the best or most salt inducing custom moves appear to be Kong Cyclone, Extreme Judge, Relentless Missile, Luma Warp, Shifting Shuriken, and a few others I guess I can't think of off the top of my head. Suffice to say, though, the community as a whole has had a good amount of time to examine these custom moves, and not a single one of them has been determined to be degenerate to gameplay or tests of skill in any way. Each individual custom special move has its own tradeoffs, with some definitely being better than others, but they can all definitely be dealt with in some way. There doesn't seem to be an exception to this right now.
The other big complaint appears to be that it's too hard to learn and remember each individual custom set a character might run(This can also parallel as an argument for Mii Fighters' size discrepancies). For both sides of the argument, this is obviously just a subjective personal point. Regardless, I believe that the "breadth" of matchups provided by all of these custom moves helps add to the game's depth and overall complexity. Having a more varied selection of options for yourself and having to understand the different kinds of options your opponent possesses is something that I truly believe can be part of a legitimate skillset for this game.
I believe that Smash 4 has the potential to become so much more with custom special moves thrown into the mix. That is my belief.