It's simply called RNG (Random Number Generator) for a reason. Randomness is NOT supposed to be balanced what-so-ever. That's why it's random. Duel Links is NOT Rigged. I repeat, Duel Links is NOT Rigged.
Seems each LD has some SR card that harder to get than the UR one.
For example :
Kaiba - E-con
Mako - Forgotten Temple of the Deep
Yugi Muto - Union Attack
Arkana - Anti Magic Arrow. Even until this day Mako never drop Forgotten Temple to me
For me, the cards that were (or still are) just plain hard to get: Ishizu-Zolga, Mai-Amazoness Tiger, Odion-Millennium Scorpion (don't care about this card, but he still won't drop it), Yami Bakura-Dark Voltanis (farmed this b**** like a million times and got everything else under the sun except this). lol