1. Electro-magnetic fields (EMFs) from the electronic equipment creates an unnatural space for rest. If you cannot remove the equipment, at least unplug it, since EMFs still flow even when the item is off. (For more on EMFs see this article).
2. Yin vs. Yang – Yin refers to things that are softer, quieter, calmer and yang refers to more loud, vibrant and energetic. What type of energy is best for a bedroom? You've got it…yin. Electronics are yang energy, adding to noise, clutter, and most have reflective surfaces that act like mirrors, bouncing energy around the room. Hiding the items behind a cloth can help minimize the yang energy.
3. Primary purpose – What is a bedroom for? Well, it should be for rest and intimacy. That is it. A bedroom is not for television viewing, book reading, or, god forbid, doing your bills or work! It's also not for exercising. So why are these items in your bedroom space? If you have a very small home and no other place for them, then you need to hide them when not in use.
So I hope that explains it. If you live in tight quarters, then you might need to have some electronics in your bedroom. If you can avoid it, then avoid it. Removing electronics is much better than covering them up.