Why do I get earwax in only one of my ears?

Ear Wax. Both ears are making wax. If you have damaged the skin lining the ear canal (with q-tips), the wax may not migrate on one side as efficiently as on the other - causing a build up.
Sleeping on it. If you sleep on your ear it can prevent the ear from clearing and can push the wax in. If you use a phone in only one ear and press it to the ear this can also cause one sided blockage.
Every ear different. Ears are all different, some produce more wax than others or are narrower or wider than others. Both ears produce wax, as is normal, you are just noticing it more on one side. Nothing to worry about, unless you get infections related to this (foul smell, liquid drainage or crusting, or hearing loss).