The main reason for this happening is that hotel room key cards hold very little information on their magnetic strip (around 2% or so), whereas other cards (credit cards, DLs, etc.) hold more information on them. Something about what's encoded on them has to do with how "durable" it is, thus making your permanent cards less likely to be decoded or damaged. If you swiped a credit card for all its data in a card swiper, lots of information comes up. Swiping a room key card pulls nothing off the strip.