This is how Blizzard controls the luck of players. Buy lots of stuff in the store and your RNG flag will improve.
After helping my friends earn a dozen mythics (and me getting zero despite play 4x as much and helping level them all), I finally broke down and did the pre-purchase. Minutes after my visit to the store, the next uber boss dropped two mythics – the next few runs of Duriel dropped two more; bringing my Mythic inventory up to par with the buddies (except one, who buys every trinket he can from the store and has exceptionally great luck with gear drops).
But no – the game is NOT pay to play.