Card magic is easy in the sense that it is accessible and familiar to everyone. If I (in a general sense) tried to specialize into more general magic with more props, it's more likely the audience believes the props are gimmicked in someway, diminishing the magic of it. However, cards and coins seem to have a sort of innocence surrounding them: everyone has access to a deck of cards or some quarters and so the audience is less inclined to believe you have some trick up your sleeve. Sponge balls, cup and balls, rope all have a connotation of being specifically for magicians while a deck of cards is more relatable to the everyday layman. A lot of people don't know about the existence of gaff cards, knowledge on duplicates, etc., because cards are so common throughout our everyday lives that we all just assume that every deck of cards we see is normal, like a standard deck of Bicycle playing cards. If I tried to perform magic with other things that weren't cards as well, I wouldn't be able to perform as many effects. The versatility of card magic means I can learn a plethora of effects without having to invest in too many props: I spent the first two years of my magic experience with one incomplete deck of Bicycle playing cards and a few YouTube tutorials, books, and online instructional videos before I finally bought an actual deck and some other things to progress in magic.
Tl;dr-cards are relatable, easy to access, and versatile in magic.