Seems like bad luck / bad RNG?
On the one hand, it only took me 4 or 5 tries to get the Epic (blue card) out of the silver (15000 cash) box for the event quest. Probably very lucky on my part, I'm typically seeing 1 out of 20 or so like you.
On the other hand, I've only gotten one legendary (red) card out of the gold (50 gem) box the whole time I've been playing. Which is probably bad luck on my part, but like you I've only been grinding free gems from daily logins so that's probably only 10-12 boxes total.