In a nutshell, it is the price you must pay for requesting a single cabin, which seems like a penalty for cruising solo. In fact, that's exactly what it is, because the cruise lines want to make up for the revenue lost by that empty bed in your cabin. And not just for the price of that cabin -- one less passenger onboard means one less person buying drinks, souvenirs, shore excursions, etc. So the cruise lines make up for that, in part, by charging that dreaded "single supplement."
Some ships do have cabins for single-occupancy available. These are more common on older vessels due to irregular cabin configurations that were designed to fit the shape of a ship's hull. Cabins on contemporary ships are mostly uniform, constructed in a modular design and installed after the hull is afloat. So, with most cabins designed for two people, the cruise lines charge the supplement, which is generally stated as a percentage of the per-person, double occupancy rate.
One way around paying the single supplement is the "guaranteed share plan," something that is offered by several cruise lines. You agree to share a cabin and are charged the per-person, double occupancy rate. If no cabin mate is found, you have it all to yourself and do not pay anything extra. Be aware, however, that you will only be matched by gender, not age or smoking preferences. Sometimes the share plan works quite well, and you might find a lifelong friend, as well as a future cruising companion. On the down side, you might be so miserable you will want to sleep outside in a deck chair, because rooming with a stranger is always a crapshoot.
The policies of the major cruise lines as to the supplements they charge and whether they offer a guaranteed share program can vary, and they may even change from time. When booking, always ask about any specials currently being offered. Regent Seven Seas recently had a rate of only $99 for their single supplement. In addition, rates will sometimes vary according to category, and prices may also drop close to sailing date, because ships do not like to sail with empty cabins.