Apart from being by far the best But the main reason in my opinion is that it has pretty much no deckbuilding requirement. It simply has the most gold cards to trigger it, a lot of them being staples of the faction anyway and borderline autoinclude in every deck.
But let's just put it into numbers:
NG has 14 gold aristocrats (15 if you count copying with Coup)
SY has 7, about half of which are trash
NR has 3, none of which see any play
SK has 7, all highly archetype specific
ST has 16 but most (especially the good ones) are highly archetype specific
MO has 11, which are by nature of deathwish cards highly archetype specific
The consequence is that in a lot of cases you don't build a deck around Ball but rather build a deck, notice you need a bit more firepower and figure "ah, what the hell, I'll just throw in Ball, I already have aristocrats anyway".
It's the equivalent of giving SK a warriors scenario or NR a soldiers scenario.
In other factions you have to build your deck around the scenario and commit to it. In NG you don't. It's simply way too easy to trigger. In my opinion that's a problem.
Source: https://www.reddit.com/r/gwent/comments/m1gddu/the_simple_reason_why_ball_is_by_far_the_most/
