We finally got an opportunity to go to San Francisco and the highlight of my trip was to be a tour of Yosemite. We found "Incredible Adventures" in the Bay Guide. I made reservations 2 months before our trip, called 2 weeks before and confirmed reservations, called them when we got to S.F. on a Monday for a Saturday tour confirmation. Guess what - Sat. AM we were in the lobby awaiting pickup - and no one showed up. Called the agency 15 minutes after pickup time and was told that "somehow our pickup day had changed (since that Monday). David couldn't explain why or how this happened, but he "was sorry". That's it - my probably once in a lifetime chance and it was screwed up and I was given an "I'm sorry". Of course, I guess, after the fact, that this was all he could do - but that didn't help. Just thought you might want to know! "Yosemite-less in S. C."

That's just awful!!! I think you should definitely send a letter to the tour company with a carbon copy to the Better Business Bureau and the San Francisco tourism office. I know it won't help you see Yosemite, but maybe it will help others avoid this company. That was inexcusable behaviour. Especially after you re-confirmed twice. Unbelievable!!

Shirley, I am so sorry that happened to you. Yosemite is one of my favorite places on the planet. I hope you get another chance to come and see the park sometime.

I second Beth's suggestion that you write a letter. I would also CC the mayor's office as tourism seem to be one of his priorities for the city.

Again, so sorry to hear your trip was ruined.

See this US post,

Results of complaint to 4 Seasons Hotel

I have seen this happen before in other cities. They consider a reservation is for the tour company to plan where they are needed. Not as a gurantee to the customer that they will depart on that particular day and time.
Hope these next comments help with your future plans. When I travel alone, I book day trips with small tour operators, where they may fill a van with up to 12 people (sort of like Super Shuttle from the airports) On two occasions, in San Antonio and Santa Fe my tour was canceled because they needed a minimum of 4 or 6 people to make it worthwhile. Each time I expected it. I have learned to book my tour for as early in the week as possible so if they have to change it, I still have a chance of going. Chances are your Yosemite tour would not have gone on Monday either, If they can't fill up with enough people on the weekend it is unlikely they would on a Monday. When I book a tour I ask in advance what the minimum requirements are for going, and if anyone else has booked it the week I am in town, and suggest if anyone inquires about tour departures that week that they mention someone is all ready booked for tuesday or whatever day it happens to be that I booked. I was lucky in San Francisco, I asked the tour company which day had the most people booked for Yosemite, when they told me 4 people were all ready booked for Thursday, that was the day I booked. They did show up on time at my hotel, we made stops at the other two hotels, and off we went. I am sorry you missed this opportunity. One other suggestion I would like to make is give everyone you deal with the name and number of the hotel you will be staying in. I have had good fortune with US Airways, calling me to advise of delays (thunderstorms: the plane never left it's previous destination, so it would not be arriving where I was to pick me up)and with reroutes from Continental and US Airways. Maybe the tour company could have taken you earlier if they knew how to reach you.
One last comment, I suspect the tour company advertises about four day trips, wine country, muir woods & sausalito etc. chances are they only have one or two vans and whichever tour gets the most request is the one they do. This has happened to me twice.