BO1 is popular because BO3 isn’t supported Recently, wizards’ marketing sent letters to LGSes asking them to support BO1 during game nights because ‘it is so popular on Arena.’  

I submit that BO1 is so much more popular because BO3 has zero support from ladders or pre-season ranking.  If BO3 was where we earned ladder rank and BO1 earned nothing, I suspect the numbers would be markedly different.