Bandwidth sharing isn't quite the panacea that a lot of people think it is.
For starters, you have to get the end user to download and install a piece of software first (torrent client, etc) and then direct them to the larger downloads of the actual software you're distributing.
Next, you can't just build this feature into something like our PortableApps.com Platform (with it's built in software downloader/app store/updater) as many end users are prohibited from running P2P software by their ISPs and we can't depend on them being technically knowledgeable enough to know whether or not they're allowed to.
Then you have the issues with routers and firewalls and punching holes in them to allow people to upload to others as well as download (which is a bit easier with upnp but not always automatic).
Then you have the issue that much of the world is still on metered connections. Here in NYC I have the choice between slow DSL provided by one company (and some resellers that use the same line), fast but unreliable cable provided by one company (no resellers or competitors), no fiber (Verizon cancelled FiOS buildouts), or wireless (which carries a limit of 5GB per month and you pay $10 per GB after that). I opted for the fast but unreliable cable and a wireless hotspot as a backup for the several hours a month the cable goes down. Lots of the world has even fewer options than I do.
And finally, most legitimate webhosts prohibit any kind of P2P hosting on their networks, so you can forget about running a torrent tracker on your regular web server. You can go with a second tier provider that is more forgiving (or clueless) but then you have the issues associated with such a provider (likely illegal activities on the same network, likely security issues, etc).
There are other issues and some workarounds for the above, of course, this is just my personal experience with researching it for PortableApps.com over the years.
Some parts of the problem sound like they could be described as a tragedy of the commons. Things would be more efficient all around if we just allowed intelligent bandwidth sharing through P2P, but network administrators concerned about the impact on their particular networks prevent such moves, actually making everything less efficient.
For starters, you have to get the end user to download and install a piece of software first (torrent client, etc) and then direct them to the larger downloads of the actual software you're distributing.
Next, you can't just build this feature into something like our PortableApps.com Platform (with it's built in software downloader/app store/updater) as many end users are prohibited from running P2P software by their ISPs and we can't depend on them being technically knowledgeable enough to know whether or not they're allowed to.
Then you have the issues with routers and firewalls and punching holes in them to allow people to upload to others as well as download (which is a bit easier with upnp but not always automatic).
Then you have the issue that much of the world is still on metered connections. Here in NYC I have the choice between slow DSL provided by one company (and some resellers that use the same line), fast but unreliable cable provided by one company (no resellers or competitors), no fiber (Verizon cancelled FiOS buildouts), or wireless (which carries a limit of 5GB per month and you pay $10 per GB after that). I opted for the fast but unreliable cable and a wireless hotspot as a backup for the several hours a month the cable goes down. Lots of the world has even fewer options than I do.
And finally, most legitimate webhosts prohibit any kind of P2P hosting on their networks, so you can forget about running a torrent tracker on your regular web server. You can go with a second tier provider that is more forgiving (or clueless) but then you have the issues associated with such a provider (likely illegal activities on the same network, likely security issues, etc).
There are other issues and some workarounds for the above, of course, this is just my personal experience with researching it for PortableApps.com over the years.