Or just plainly logical. Speeding up time is all well and good, but why would it make someone discard cards? What exactly is it the Elemental is throwing away, and why?
As I see it, an Elemental has to harness quanta from natural energy around them. Pillars/Pendulums attract this energy so they can siphon it. They then use this quanta to form material things, but they have to prepare this process beforehand.
Before going into battle, they have to prepare different methods of shaping quanta. This shaping could consist of geometric shape, colour, taste, smell, texture, etc. All the senses and probably several more we humans aren't capable of. They embody their element, after all, and thus probably feel the energies around them in a way we could only begin to describe.
These methods for shaping quanta would be what represents cards for us. A general idea on how to form the quanta, how to mould raw energy into something they can use in battle. They have to have prepared this beforehand so it could be done almost instantly, and forgetting how to mould the quanta into one specific individual spell/permanent/creature would be the equivalent of discarding a card. When the elemental runs out of prepared methods, it has no way of fighting back, and surrenders.
It's somewhat logical, but only one way of looking at it.
Edit: Hm, I forgot about the mark. I guess an Elemental generates quanta in a similar way to how we generate heat. That means they need a source of energy to do so from. Perhaps they work like a star, constantly burning the pure energy they are made from. Many possibilities. x)