Alot of new Players dont know it but you can only drop certain cards from daily Quests or end of Season Rewards.
Currently there are 21 different Cards(3 Legendary,4 Epic,8 rare and 6 common).
That also explains why certain Legendarys are cheaper than other Legendarys.
below you can see The Legendary cards that currently drop.
If you want to know all the cards you can drop and how many cards already drop cheackout the Link
https://kiokizz.github.io/Splinterlands/cardsByLeagueCap.html