The issue of the single supplement continues to vex solo travellers but too often it's the tour operator copping the blame, and that's not fair. The reason the single supplement exists is due to the way hotels charge for their rooms, and it goes into the pocket of the accommodation provider, not the tour operator.
In busy times and in popular destinations, hotels charge one price for their rooms. Doesn't matter whether there is one person or two in the room, the room rate is the same. If the tour operator was to charge the same price for each client regardless of whether they were travelling solo or as a couple, the operator would either lose money with every single traveller or else they would jack up the tour price so that two sharing a room are subsidising those who choose to travel solo.
You can sometimes find tours without paying a single supplement but that will usually happen only if you're travelling in a quieter time, or to a less popular destination. Most tour operators will try to match solo travellers with another and therefore avoid the single supplement but if you insist on a single room, expect to pay more. However, those who pay a single supplement are entitled to a decent room. Too often, and in Europe especially, single travellers are shunted into small rooms with the worst outlook. If that happens to you don't let annoyance fester, complain about it straight away with your tour leader.