Its not about pets, it about 2 toys, the transmog beacon and the fireworks.
Each of these toys was prices at $10 dollar and 25% of that would go to the MDI and Blizzcon WoW Arena Championships, to thats $2.50 per toy addin to the prizepool.
In the end the prizepool was $660000 ( https://worldofwarcraft.com/en-us/ne...ls-at-blizzcon )
So thats 660000 / 2.5 = 264000 toy sales to generate that prizepool. Now i don't know what the conversion rates are for mediocre toys to sell like this, but i doubt more than a few % of the playerbase would buy stuff like this.