I don't want to come across as religious, but you do get most of that stuff out of the box with git+github. The main feature of Fossil seems to be it's default auto-sync mode. Which could be set up with git, but it would require some effort.
Having a DSVC+online issue service is not the same at all than having all in one package. It's the whole point of Fossil (it's explain in the whole website).
First, you can work with the wiki, issues and other project documents without being connected.
This is incredibely important for some people (like me) who wants to be able to work offline without having to get online to update information (that would be lost anyway because some informations will be forgotten or badly transcribed later).
Second, what if you don't want to make it public?
Then github/bitbucket/googlecode are not a possibility. You have to setup you own services, on a private server, or have to use something simpler.
Third, assuming you can setup your own service, it will be long, hard and require tons of knowledge that are not required for the project itself (in particular for most non-web based games).
Fourth, what if you don't want to put you project online at all. I have several projects that I like to work alone on my computers or with some friends and really don't need to put it online (even if I have my own server online). We can easily share the source code with decentralized tools but it's not true for the rest of the tools that we might want to use. I have one project with a separate repository that have only text documents that act like a wiki and an issue system. It's ok because I'm alone on it, but once I get 2 or 3 people in this project, it can't be used.
In those conditions, fossil looks an interesting alternative to all the other dsvcs, simply because it move a lot of work out of you way.
Comparing git and fossil is a bit ridiculous to me (without trying to insult you, I just mean I don't see how you can get to this comparison) because they don't want to achieve the same thing at all. git and mercurial are about the code source only. Github is totally another thing and it have disadvantages like public code, limited issue tracking, etc. It is really nice for open source projets though, but not everybody are in conditions to use it (or bitbucket or google code) (even if bitbucket allow private repos, it's still too limited).
Fossil targets specific cases: you just want to work on your code, have tools to manage the project, not something fancy, but something that work well enough, and that you don't need to understand how to setup a web service to put on.
I've done setting up TRAC and Redmine and other for years, for different projects. I've also tried different ways to manage little projects. My current way of thinking is that there is no good tool, only paper or a short list of issues in a text file work well for a project I'm alone in. But when I have projects with several persons, I must still setup some work environement. And that's really really lot of work for not a lot in the end.
Git mercurial and bazaar doesn't provide what fossil provide. github, bitbucket, launchpad and google code provide the missing part, but as an online service. That's a particular combination that is adequate for a lot of projects, but cannot match all kind of setup.
Also, assuming you are an indie game maker, using github(or any other online service) without paying for getting privacy is just killing the sells of the project. Successful (in the financial way) project that are opensource were not when they got open source, but opensourcing did make them even better at selling afterward (I m thinking about Aquaria for example) because of popularity and enhancement like making it cross-platform with the help of the community.
Also, don't forget that when you put source code in a service like github, you don't have any control on the access of the service. It's true even if you pay for a private online server and manage it yourself. Having duplicate copies of you wiki, issues etc. and other documents of the project, like the code source, is maybe the most secure way to keep the project alive.
But not everybody needs that. Some do. It depends on the context.
I'm considering using fossil for some projects, but not all because they are not in the same context at all. Being religious about your tool is, to me, the best way to finish your evolution as a creator. (even if obviously, using the tool you like more to do the right thing it was invented for is the best way to be able to finish something).