I would argue that the tournaments you mentioned were so split apart in dates, that patches/ introductions of new characters are to blame for that vast variability, which makes it "seem" like it has more characters who have more of a chance.
Let's start off with Super Smash Con (August 6th-9th 2015):
Patch 1.1.0 which the tournament was on, seen a major improvement to DK and a minor improvement to Ness' meteor hit.
At this point: ZSS, Pikachu, Sheik, Diddy were already dominating the scene at the time, so not much variability there.
Bayonetta, Corrin, and Cloud were not yet introduced.
Ryu was only around for about 2 months at that time, so it makes sense you don't see Ryu up there.
I would argue that using this tournament as an example of variability, is a great stretch considering: All the characters were not introduced, and between this and Genesis 3 there were 3 patches introduced. So let's not consider this a "valid" tournament to analyze for variability.
With that said:
Genesis 3 January 15-17-
Corrin and Bayonetta were not yet introduced, so they can't be accounted for.
Cloud was just introduced a month ago, so that makes sense he was not seen in tournament form.
At this point: Sheik, Zss, Fox, Ness, Diddy, and DK make an appearance in Top 8 again (If you look at Super Smash Con). SO there's not much variability there. The only different this time? No Falcon or Pikachu, instead there's villager. So that's cool, Villager is learned yay! But that's about it.
Newcomer to top 8: Villager
Beast 6- February 19th-21
At this point: Sheik, fox, ness, diddy show up once again in top 8.
Note: Pikachu shows up again after missing from Genesis 3.
Newcomers to top 8: Greninja, Cloud, Rosalina
Glitch- March 12th
At this point: Sheik, Zss are still top tier and make an appearance.
Bayonetta is introduced February 3rd, and a month later, makes an appearance at top 8. Funny, how out of all the tournaments you mentioned, no other new DLC appeared after just a month. Rather it took a few months to learn...hmmmm...
Newcomers to top 8: duckhunt, robin, and metaknight
Pound 2016- April 2-3
At this point: Sheik, ZSS, Bayonetta , and Metaknight once again show up in top 8.
Newcomers to top 8: Mario, Cloud, Mewtwo
Interesting Trends based off these tournaments
Out of all the introduced DLC characters, it took Bayonetta the shortest amount of time to reach top 8. ~1 month
Sheik showed up 5/5 times
ZSS showed up 4/5 times
Bayonetta showed up 2/2 times (She was only available in those two tournaments
Fox showed up 3/5
Ness showed up 3/5
Diddy showed up 3/5
Once Genesis 3 happened, DK disappeared.
Falcon only showed up in Super Smash Con, so can he really be considered as a variable?
Conclusion:
Sheik, ZSS, Bayonetta have been in almost every top 8, so not much variability there. Can't be considered.
Fox, Ness, Diddy, have appeared 60% of every tournament in top 8. Again, not much variability there (You can say that at their times, they were considered the "Top Tier" but due to new character introduction and patches, if they started showing less results, it could possibly be due to that).
The only characters which have made a random appearance in these tournaments are: 12
Mario, Cloud, Mewtwo, Greninja, Rosalina, duckhunt, robin, metaknight, Villager, Captain Falcon, and Pikachu.
If you look at it, the "Top 6"
Sheik, Zss, Bayonetta, Fox, Ness, Diddy = 10%
Variable characters out of the roster at these tournaments = 21%
So when you really look at it, there isn't that much variability in the characters that show up in top 8. The only reason it "seems" like that, is due to the fact that
A. Bayonetta, Corrin were not introduced for most of the tournaments.
B. Cloud was still new to one of the tournaments so he had to be learned
C. Patches were introduced after Super Smash Con, which changed the metagame.
D. Even when there was variability, those characters showed in only 1 or 2 tournaments.
I don't know if you can really consider that to be more "balanced" especially considered Sheik, Zss, Bayonetta, Fox, Ness, and Diddy have shown their faces over or equal to 60% of the time. (Reminds me of Melee in that sense considering character who often show up in top places are: Fox, Falco, Marth, Sheik, and Jigglypuff due to Hungrybox) :O