Upload speeds are often that low to increase download speeds. If you have the option between 30/30mbit or 50/10mbit, most users will have a much better experience with 50/10. Upload speeds beyond 10mbit/s are rarely noticeable for standard applications of most users (streaming, surfing).

It also kind of prevents people from running servers at home but renting a server with average connection quality (what you'd get at home, no multiple redundancy and excellent peering) is not much cheaper than the energy costs of running it at home (at least in Europe). So I doubt that this is the main motivation for ISPs.