Why do most magicians show magic with cards? Because it is portable, and the magic is close to your eyes and not on a stage which is meters away! Spectators really love it when they can see something that's 'in their face'. Plus magicians love the ability to do huge magic effects with small objects. And when you want to do magic with random people on the street, it is easier to approach people with a deck of cards which is apt for their attention!
card magic for a number of reasons. Here are the ones that come to mind:
Comfort . It's likely they learned at least some card magic early in their magic career so it's something they feel comfortable and familiar enough with to perform relatively soon.
Pocket management . As a close-up magician you can't drag along large piles of props. You need to be economical about it and choose multi-purpose props you can use for multiple effects. Cards are small, light and versatile in their use.
Expectations . People have come to expect card magic from a magician, so they cater to their audience.
If you were to start out with items that people typically only see in the hands of a magician they are immediately a little more skeptical so cards are a great way to learn and you can hand them out when you are done.